Much has been said about the need for and the benefits of leveling your RV. Gas absorption refrigerators used in most RVs need to be pretty level to work properly. Modern refrigerator designs are less sensitive to being level than some earlier models but should still be level for optimum performance. "Why?" you might ask. An off-kilter fridge allows the liquid coolant to pool in unintentional low spots, blocking circulation. You need level work surfaces like counters and tables so your dinner doesn't slide into your lap. You need a fairly level bed so you don't roll out or slide off the end -- or wake up with a mouth full of bile because you were sleeping with your head lower than your stomach. Yccch!
Leveling and stabilizing are not always the same thing. Hydraulic leveling systems do both at once. If you don't have hydraulic levelers, you need to both level your RV, by digging holes for the high side wheels or using blocks under the low spots and then stabilize it with stabilizing jacks between the body and the ground.
There are several options for leveling an RV. First, always start off by parking on a fairly level site whenever you can. In primitive campgrounds or open camping areas this may mean jockeying your rig around a bit to find the best spot. Developed camp sites should be graded to be fairly level. If you have hydraulic levelers, they will compensate for some pretty uneven ground, but you may create a safety hazard if you aren't properly prepared. If the rear wheels are lifted off the ground to level a motorhome, the parking brake becomes ineffective and the unit may roll off the jacks. I've seen this happen more than once. Use chocks to block the front wheels whenever there is chance weight on the rear wheels will be reduced -- and ALWAYS start with the RV as level as possible. Sometimes, if the site is level enough, you don't need any additional leveling, but you may still want to use jacks to stabilize your RV.
There are many devices to assist you in measuring how level your RV is. One of the simplest and least expensive is a pair of bubble levels. They are usually about 2-3" long and come with self-adhesive tape to mount them to most hard surfaces. Park you RV on a level place or use a carpenter's level to make sure the RV is level. Put the carpenter's level on the floor or a counter top. Since most tables are removable or adjustable, they are not the best indicator of whether the whole unit is level. Once the RV is level, install one bubble level on a vertical surface parallel to the front of the vehicle (usually the dashboard on a motorhome). Install the other on a side wall, usually just below the driver's side window. Both should be where the driver can easily see them when parking. Another clever device that is particularly helpful when you're using leveling blocks has wedge-shaped slides on each corner of a bubble level. You place it on a countertop then adjust the slides until the bubble is level and read how much each corner needs to be raised to level the RV.
Finding the most level initial spot for trailers is usually a little more difficult since leveling the tow vehicle doesn't necessarily mean the trailer is level. Here again, bubble levels come in handy. You can buy very large bubble levels to install on the front of your trailer where you can see it from the tow vehicle so you can at least find a spot that is nearly level side to side. Usually you can easily adjust the front-to-back leveling using the tongue jack after you've disconnected the trailer from the tow vehicle.
For years I've used a device called a clinometer to help with finding the most level spot. These are sometimes used in off-highway vehicles like rock crawlers to help the driver gauge angles to avoid roll-overs etc. They have floating indicators inside that show the tilt side to side and front to back. The displays is similar to an artificial horizon in an airplane. Simply maneuver around the campsite until you find the spot where both the side to side and front to back indicators are most nearly neutral. The clinometer should be mounted where it can be easily seen by the driver but where it doesn't interfere with normal views of the road or interfere with other instruments or controls. These are most helpful in motorhomes. Making sure the tow vehicle is level doesn't ensure the trailer is level, but it might give you a starting point. You might have your tow vehicle perfectly level and still have the trailer hanging at a rakish angle out back.
Once you've found the most level spot, you may still need to make adjustments to ensure your RV is adequately level. The easiest way to do this is with automatic hydraulic leveling systems. You only need to activate them and they will automatically raise each corner of the RV until it is completely level. With a manual hydraulic leveling system you'll have to operate levers to adjust each jack but they usually have indicator lights to tell you which one still need to be raised higher. When all he lights are out you should be level. If you are parking on soft ground, be sure to put "jack boots" under each jack before raising the RV. Even fully automatic systems will lower the jacks and pause so you have an opportunity to place jack boots under the jacks before starting the jacking process. The jack boots create a larger footprint to prevent the jacks from sinking into the ground. Even if the ground feels hard to you, it is a good idea to use the jack boots anyway. I've seen the jacks on large motorhomes sink 6-8" into ground that feels hard as pavement to walk on or drive tents stakes into. Vibration from movement inside the vehicle or running the generator may contribute to sinking. Manual hydraulic levelers are functionally similar, but instead of electronic sensors that check how level an RV is and control activation of individual jacks to bring up the low corners, the operator controls each jack using switches and has to monitor the degree of levelness using bubble levels or indicator lights on the leveler control panel. A single bubble level with a circle in the center can be placed on any solid level surface and monitored. When the bubble is centered in the circle, the unit should be level. This type of level is often used to check how level the refrigerator is. If you don't have hydraulic levelers, there are usually a couple of options for leveling your RV. In primitive areas you may be able to dig holes under the high wheels to lower that side or corner. You'll have to estimate how deep you need the holes, dig them in front or behind the tires, then drive into the holes; repeat as needed. Digging holes can be a problem if the ground is wet or you get rain while parked in the hole. On pavement or other hard surfaces you'll need leveling blocks or jacks of some kind to raise the low side or corner. To use leveling blocks, place them in front of the tire on the low side/corner and drive onto them. Make sure the blocks are large enough to support the entire footprint of the tire, front to back and side to side. If you have dual wheels, use equal blocks under both tires and maintain equal weight on both tires by making sure the top of the blocks under both tires are at the same height. You can buy plastic leveling blocks that look something like Legos. They can be stacked to create various heights and to lock them to each other. Home made leveling blocks can be constructed from dimensional lumber (2x6 or 2x8). Pressure treated lumber will be more resistant to moisture and rot. One end can be beveled to make driving onto them easier and reduce the chance of the tire pushing them out of the way. Several blocks can be stacked flat to create 1 1/2" incremental height adjustments. Another option is to cut the blocks into wedges and fasten several together to make them wide enough for the tires. This gives you a continuous wedge from zero to six or eight inches, depending on what size lumber you start with. Note: finished dimensional lumber is actually 1/2" smaller than its designated size in each direction. A 2x4 is actually 1 1/2" by 3 1/2". Manual stabilizing jacks are mostly seen on travel trailers as the weight of motorhomes is usually too great for them to be effective. Hydraulic or scissors jacks may be used to lift a low side or corner while you install and adjust blocks or stabilizing jacks. Stabilizing jacks are typically the shape of a 4-sided pyramid and usually have a large bolt and nut in the middle that can be adjusted to achieve the proper height. Some height adjustment can be obtained by turning the bolt, but it is much better to use a real jack to lift and lower the vehicle and use the stabilizing jacks only for stability. There are special scissor jacks designed for leveling and stabilizing travel trailers that are permanently mounted to the unit. I've seen these used on small motorhomes, but the extra weight of the engine and drive train sometimes makes them difficult to operate and they might not be strong enough if extensive lift is required to level the unit. Anytime you can establish a solid link between the body and the ground your RV will be stabilized and won't bounce like it does resting only on the suspension.
RVs equipped with air bag suspension can sometimes be leveled by adjusting the pressure in individual bags. I once had a Pace Arrow with this feature. The classic front-wheel drive GMC motorhomes also had this feature. While this is a lot more convenient than kneeling the mud to place leveling blocks, using the air suspension does not stabilize the unit like hydraulic levelers do. The body can still bounce on the suspension. The amount of lift is also somewhat limited so you need to be pretty level to start with. Unless you are particularly sensitive to bouncing (or especially rowdy!), leveling with an air suspension option is usually quite satisfactory, but you will still get some rocking on the suspension from moving around inside and/or from cross winds. Hydraulic stabilizing jacks brace the frame directly to the ground, taking the suspension out of the equation.
Bubble levels mounted on the dashboard and on the wall to the left of the driver are inexpensive and are easy to use to find the most level spot on a camp site. The larger ones can be used on the front of trailers where they can be seen from the driver's seat of the tow vehicle . Clinometers provide a quick estimate of positioning while parking. I read of an RVer who mounted an inexpensive ($1) torpedo level on the running board of his Class C motorhome. He could open the door and check the front-to-back level status and make drive forward or back before he shut off the engine. If you find yourself in camp without any levels, you can still check how level your RV is by putting some water in a cup, bowl, or pan and setting it in a counter or table top. When the water is equidistant from the top of the vessel all around, the vehicle is level -- assuming the surface you put the vessel on is level. For that reason I suggest using a fixed countertop. Tables, which are usually removable, and cup holders may be out of kilter even when the vehicle itself is level.
Home made leveling blocks are inexpensive and can be easily customized for your particular use. They are usually made of 2x6's or 2x8's and beveled on one end so the tire will roll up onto them instead of pushing them out of the way. The most common pattern is to cut several blocks and stack them as needed for additional height. You might add an extra block on the top board to serve as a chock to prevent you from driving off the blocks. An alternate design is to select several pieces of 2" lumber together until you have a total width slightly wider than your tire tread, then cut the blocks diagonally and fasten them together to create two wedges. This gives you a continuous slope for adjustment instead of going up in 1 1/2" increments you get with stacking blocks. The only downside to this type of blocks is that they are somewhat large and it is sometimes difficult to find a place to store them. But they give you smoothly adjustable height for precision leveling.
Always chock your wheels. You never know when someone might bump the parking brake and release it. I've even heard of small pets hitting the parking brake and releasing it on some units. Because uneven compression of soil can let blocks or jacks shift, vehicles sometimes have a tendency to roll if the wheels are not blocked. Travel trailers with tandem wheels sometimes use a wheel lock that consists of two wedges that clamp between the tires. An adjustable screw or clamp between them tightens and holds them into place. This locks both wheels and keeps the trailer from rolling in either direction. While it would be safest to chock at least one wheel both front and back, in most situations you only have to worry about a vehicle rolling in one direction so placing chocks on the downhill side of tires in contact with the ground is usually sufficient. Some leveling block systems include chocks that hold the tires in place on top of the blocks. The blocks themselves need to be significantly longer than the footprint of the tire in order to have room for chocks.
Note: leveling blocks used under the tires (and air ride levelers) will NOT stabilize the body. To stabilize the body, use stabilizing jacks under each corner as described above. Hydraulic leveling systems usually do stabilize the body by bracing the frame directly to the ground. If you level your RV only using blocks under the wheels or finding a sufficiently level place to park, the body will still have a tendency to bounce on the suspension whenever there is movement inside or a breeze outside. Scissor-style leveling jacks used on many trailers also provide stability by bracing the frame to the ground. Your RV may be perfectly level when parking in a developed camp site, but without stabilizers between the frame and the ground, it may still be subject to an annoying bounce or tilt due to wind or to movement inside. I once found myself parked with the broad side our Class C motorhome facing a strong wind. I spent most of the night lying the shaking and pitching cab-over sleeper trying to figure out what to do if we got blown over! Of course stabilizing your RV may negate the value of the bumper sticker that says "If this camper is rocking, don't come knocking!"
Stabilizers in the form of Straight jacks or scissor jacks are sometimes attached to each corner of a travel trailer. You will need a total of 4. Sometimes you can used the tongue jack of trailer to stabilize the front and just two jacks on the rear. Jacks can assist in minor adjustments in leveling and will connect the body to the ground to prevent it from bouncing on the suspension. Manual stabilizer jacks are not usually used on motorhomes, mostly because of the added weight and reduced clearance. Portable stabilizing jacks can be used on any RV. They consist of aluminum jack frames with adjustable steel bolts in the tops. The adjustable bolts let you adjust the height of the jack until it firmly supports the RV frame. Stabilizer jacks and jack stands are not designed to lift the weight that is often needed to level a vehicle on uneven ground. They are only intended to stabilize the body to the ground to prevent bouncing.
Straight stabilizer Jacks Scissor Jacks
Stabilizer Jack Stands
Making sure your RV is level and stable will allow you to enjoy your stay, safe from pooling coolant in your refrigerator, free from rolling out of bed, protected from objects rolling off the table, and relieved of bouncing due to wind outside or movement inside the vehicle.