OK, what the heck IS "winterhoming"? Well, simply put, it is motorhoming in the winter.
Most camping is done in the summer time, or at least spring or fall, but there are sometimes good reasons to use your RV in the winter too. An RV makes an excellent chalet or base camp for skiing, snowboarding, ice fishing, and other snow based activities -- if is is properly equipped. Most RVs made and used in the U.S. are not designed for winter use and most owners who live in cold country have to winterize their rigs and retire them until warm weather returns. However, it is possible to upgrade many RVs for winter use. BTW, some Canadian made RVs are more likely to come from the factory equipped for winter weather.
While the comfort of the occupants is, of course, the major function of an RV in winter, the biggest problem is usually protecting the plumbing against freezing. Sometimes an RV furnace may not be adequate to maintain a comfortable temperature inside. If that is the case you either need to reduce heat loss or increase BTUs. It usually isn't very practical to increase the insulation factor in an RV, but a lot of heat may be lost through drafts and through insufficiently protected windows. I once had a Class B van conversion that, I found, had NO insulation at all and it was fairly easy to remove wall and ceiling panels and cut styrofoam panels to fit each space, significantly improving insulation. But it is usually too intrusive and too expensive to increase the insulation in standard RVs. Sometimes you can gain access to interior paneling on external walls to add foam or bats of insulation, but usually it would be way to destructive, expensive, and counterproductive. What you can improve fairly easily and cheaply is the insulation value for the windows. First of all, make sure you take advantage of whatever window coverings you have -- close the curtains, drapes, or shades. You can add reflective foam insulation similar to windshield sun screens between the window coverings and the windows. You may want to try adding plastic "storm windows" over the windows. You can get kits to do this from your local hardware store or home center. They consist of a plastic film that is stretched over the windows and then tightened using a heat gun or hair dryer. Also install a cover or foam pillow to block the loss of heat through the roof vents. Even when they are closed, the thin cover allows a lot of heat to escape. Search for and seal off any drafts where cold air enters through the firewall of a motorhome or around plumbing and power cords or around doors and windows of any RV. If after doing all of this your furnace still doesn't keep things warm enough, you may need auxiliary heat. Options include electric heaters if you stay in campgrounds with electric service, a catalytic heater, adding another furnace, or upgrading the existing furnace to one with a higher BTU output. Upgrading or adding a furnace can be an expensive proposition and is likely to require significant modifications for installation of a larger unit. Catalytic heaters don't use any battery power since they have no fans and portable versions attach to 1-lb propane cylinders to they don't require any gas line attachments. Keep in mind even heaters rated for indoor use will consume oxygen even if they don't release any toxic fumes, so proper ventilation is critical. Seems counterintuitive to open windows when you are trying to heat a space, but you will suffocate if you don't! Check inside cabinets for openings around plumbing where cold air might get in. These can usually be sealed easily by stuffing foam into the gaps or by using spray foam insulation available at any home center or hardware store. Take care not to over do it with the spray foam. That stuff expands a lot and is difficult to remove if it gets on surfaces where you don't want it. Make sure the weatherstripping on your doors is in good shape and that the caulking around windows hasn't shrunk or have any gaps.
Electric heaters are an easy way to get extra interior heat -- if you have shore power or when you can run your generator. A popular option among many RVers is an "electric fireplace" that not only provides heat but adds a kind of cabin-like ambiance. I even have a small, 300 watt heater that runs on 12 volt DC I can use in my motorhome in a pinch, but I'm sure it would drain the batteries pretty quickly. I found a neat little 120 volt heater that plugs directly into a wall outlet so it takes up little room. The digital temperature control makes it easy to maintain a comfortable setting in a small space. Auto parts stores sometimes offer 12-volt powered heater/defroster units that plug into the 12-volt receptacle (i.e., cigarette lighter) and can help clear fog or frost off the windshield. These units will provide a small amount of auxiliary heat but would not be very effective in as room heaters. If you have 120 volt power available you can also protect your holding tanks with specially designed heating pads and protect plumbing in exterior cabinets with a 100 watt incandescent bulb (if you can still find one! They are being phased out due to energy concerns). There are 12-volt holding tank heaters too, but, again, I would be concerned about them draining the battery. Any electric heater is essentially a dead short. The heating elements are high-resistance wire.
Protecting exposed plumbing from freezing can be an onerous task. Keeping the inside of your RV at 40° or better will normally protect all the inside plumbing, but exposed holding tanks, valves, and pipes are still vulnerable. You will need electric heating pads and or heat tape to protect these components if you are camping in sub-freezing weather. Heating pads for holding tanks are available in both 12 volt and 120 volt versions and some include dual power sources. With dual power you can use your 12-volt system while traveling and power is available from the vehicle alternator. 120-volt operation requires shore or generator power. You might run your generator while traveling to operate 120-volt heating pads. It wouldn't consume any more energy than running your A/C on hot days. If you have 12-volt heating pads you will want to carefully monitor your battery status. They could easily run batteries down if they kick in at night when you turn off the generator. One way to protect holding tanks for occasional winter use is to add enough antifreeze (marine/RV grade for potable water systems) to at least protect the dump valves and lower the freeze point within the tank contents. If you rely on warm blankets or sleeping bags instead of your furnace to keep you warm at night, dump a cup or two of antifreeze into each drain to prevent the P-traps from freezing. You don't need antifreeze in the P-traps if your furnace keeps the interior above freezing. If your RV has enclosed holding tanks and the dump valves are in a cabinet instead of hanging exposed beneath the vehicle you may be able to keep the valves from freezing by placing a 100-watt incandescent light bulb in he compartment. Finding a 100-watt bulb these days might be difficult as they've been phased out for environmental reasons. Using a 60-watt bulb might be sufficient, but using two 60-watt bulbs would be more than equivalent to a 100-watt bulb. Or you might use special reptile heat lamp available at pet stores. In any case, if the compartment isn't insulated, insulate it. Outside compartments often have bare aluminum doors that allow a great deal of heat to escape. Glue some styrofoam panels inside or even use reflective foam insulation like Reflectix. Check for drafts around electrical and plumbing connections or around the door. Random openings can be filled with spray foam insulation or stuff with fiberglass batting. Poorly fitting doors may benefit from the addition of weatherstripping. If you are connected to city water in freezing temperatures you will need a heated hose and will need to protect the faucet. Even "frost proof" faucets will freeze if a hose is left connected. It prevents them from letting the water drain out before it freezes.
RV skirts that surround the bottom of your RV can help keep the floors warmer and conserve heat. They usually need to be custom made. Vinyl skirts, made in several sections, can be carried on trips and installed when needed, but they can be pretty expensive. Some people make skirts of plywood or styrofoam panels when the unit is parked in a fixed location for a while. Skirts are usually not practical for short stays or frequent relocation.
Resource conservation will be more difficult when it is cold. You ARE going to use more propane and use it quicker than in warmer months. You might reduce how much you use for heating by dressing warmer so you can be comfortable at a lower temperature. Sweaters and thermal underwear can increase you comfort level in a cooler environment without being too cumbersome. Warmer quilts and blankets or a winter sleeping bag can help keep you cozy in bed. But your furnace isn't the only appliance that will use more propane in colder times. Your hot water heater will need to work harder to maintain normal temperatures when it is cold outside and you are likely to use more gas for cooking simply because you will want more warm foods and drinks in cold weather. The one appliance that might actually use less propane in cold weather is the refrigerator, but probably not significantly less since you are probably keeping the interior around a comfortable 72°. Always make sure your propane tank is full when you leave on a trip. For extended cold weather outings you may want to invest in an "Extend-a-stay" system that allows you to connect to an external propane tank to supplement your on board supply. For trailers with removable propane tanks you can just bring a long a couple of extra tanks.
Winterhoming is cool!