Most RVs and SUVs come from the factory with roof racks for extra carrying capacity. If yours doesn't have one, it can probably be added. However, unless your RV already has a reinforced roof adding a roof rack might not be a good idea. Without a reinforced roof any cargo you put on the rack may damage the roof. SUV roof racks that mount on the drip rails of standard cars and trucks usually distribute the load sufficiently that you don't have to worry about reinforcing the roof. But not all vehicles these days have drip rails. Be sure to check with the dealer before adding a roof rack that bolts through the roof of an SUV. You may need extra mounting plates or other reinforcements for it to be safe and avoid damaging the roof. And mounting it is not trivial as it involves removing and reinstalling the headliner, a task that requires a certain amount of experience and expertise and usually some special tools only found in upholstery shops.
RV roof racks usually include ladders to make it easier for you to access them. Be sure to examine the ladders and the racks from time to time and to tighten all the fasteners. A loose rack or ladder can be dangerous.
When using a roof rack on either an RV or an SUV, carefully consider the weight of items you put on it. Overloading the rack will damage the rack and probably the roof. Besides that, getting heavy objects up on the roof may damage YOUR body! Even if you have help getting heavy stuff up there, you run the risk of overloading the roof and creating handling problems from altering the vehicle center of gravity. Roof racks are best for fairly light loads. I've heard of guys putting a spare motorhome tire on the roof rack. T his is a very bad idea. First of all, they are heavy. Secondly, people have a tendency to drop them instead of carrying them down the ladder which can damage the tire -- or anybody or anything it bounces onto! Third, carrying a tire on the roof exposes it to a lot of sunlight which will accelerate weather checking. By the time you need the tire, it might be useless. Some tire companies actually use the roofs of their factories and warehouses as extreme laboratories to test tires under UV exposure.
If the roof of your RV is already reinforced but there is no roof rack and ladder, you can add one or have one added. If you choose to install it yourself you will need to make sure the mounting screws go through into rafters and studs or other solid support and not just the outer paneling. Likewise where you install the mounting screws for the ladder. The installers at the factory have the advantage in knowing where all the supporting structure is located. Without blueprints or a knowledgeable technician, you'll have to figure it out. Unlike residential structures, you can't count on a standard 16" center for locating studs. You might be able to locate structural members using a residential 'stud finder' or by simply tapping on the surface. If you have to resort to tapping, don't tap too hard. You don't want to damage the finish. Aluminum siding is easily dented and fiberglass can crack if you whack it too hard. If worse comes to worse you may have to drill some small holes to verify the location of solid structure before anchoring the roof rack or ladder. If the holes don't find solid structure, fill them with silicone or Bondo.
If the roof is not reinforced, you might be able to add your own reinforcing on top of the existing roof. For best results you should use marine grade exterior plywood and screw it securely into the existing roof infrastructure. This is an area that might best be done by a qualified RV technician. I would caulk the edge to prevent moisture from getting underneath it and make sure all the screws are caulked or sealed. For good looks and weather protection you might paint the plywood or cover it with a fiberglass or aluminum panel or with rubber roof membrane. You might use a roof sealer like Snow Roof instead of just painting it.
When using your roof rack, always anchor items securely using ropes, rubber straps, or bungee cords. You don't want things bouncing or blowing off the roof. Even if you think something is heavy enough that it won't blow off it should be strapped down so it doesn't slide around and damage the roof or slam into the rack and break it or bounce off if you encounter rough roads. Elastic tie downs, such as rubber straps and bungee cords are usually easier to use than rope but properly tied off rope will keep a load more secure. You can use a taught-line hitch or a truckers' haul knot to pull ropes tight to secure loads. A trucker's haul or trucker's hitch acts like a crude block and tackle to let you easily apply plenty of force to snug things down.
When using roof racks you may want to use a cargo bag or a tarp to further protect your goods from weather and sunlight. A cargo bag is usually a little more secure and weather resistant, but a tarp is often easier to use, especially when trying to cover odd shaped items. Using a cover or a tarp also helps anchor items, diffuses air flow, and reduces the chance of something falling off.
The additional carrying options you get with roof racks can be very helpful. I once added a roof rack over the cab of my Class B van conversion. The van had a pop-top that took up all of the roof over the original cargo space. I bought roof rack off a station wagon from a junk yard and cut it down to fit the cab of the van. It didn't add a lot cargo space, but it did add some. I could carry a couple of small suit cases or a tent and some sleeping bags on the little cab rack. If you go this route on any vehicle, be sure to get the "skid ribs" as well as the rack itself. They help distribute the load on the vehicle roof and reduce the chance of it being scratched or dented by cargo you put in the rack. Be sure to seal the screws or bolts you use to attach it to the roof so it doesn't leak. While you have the headliner out to install the roof rack might also be a good time to increase insulation to reduce heat transfer and outside noise.
Roof racks are not usually added to sedans and coupes, but there is no physical reason they couldn't be. They used to be quite common on station wagons and I've seen them occasionally on the trunk. Having a rack on the trunk lid brings it down to a level that is easier to load and unload but it makes getting into the trunk difficult or impossible when the rack is loaded.
Few truck campers have roof racks. Truck campers are already top heavy and placing cargo on the roof makes them even more so. That being said, it is sometimes done and requires the same installation considerations as any other RV. I've even seen hay bales carried on truck camper roofs and it looked pretty precarious to me
Roof racks are seldom seen on pop-up trailers or tent trailers. Because the roof is movable and has to be lifted into place, it isn't very practical to load it down with extra weight. That was why I put the roof rack on my Class B pop top over the cab. That being said, I have seen it done. It just means you have to have the strength of Arnold Swartznegger or take everything off the rack before you lift the roof when you get to camp. I've seen several tent trailers with bicycles racks on top. It may not be the most convenient of solutions, but if you have no alternative it might be worth considering. You'll need to unload the bicycles to lift the top to use the trailer.
Roof racks are seldom seen on sedans, but there is no reason they couldn't be added especially if you plan to use a roof tent. You may see luggage racks mounted on the deck lid, but carrying anything there makes accessing things in the trunk difficult if not impossible.
If you have a roof rack on on your SUV you might want to consider using a a car top tent instead of sleeping on the ground. Compared to being on the ground is is like having your own private penthouse, getting you away from bugs and snakes and other crawling critters that might invade your tent on the ground and giving you a room with a view. Not sure how they stand up to wind and whether having a gap between the tent floor and the vehicle roof contributes to heat loss. Something you might want to think about and prepare for if you are concerned. I''m thinking guy lines to the ground should address any wind stability issues and perhaps a foam pad under the tent floor will mitigate any heat loss problems.
Rack 'em up!