If a trailer, crawl underneath and check out frame details. I did that at a show once along with my camera. One sales dude came running over all freaked out like I was planting a bomb or something. :E
Look inside cabinetry and see how well they are put together. Look at how things like dinette seating and trim is installed. Are hinges and latches properly aligned. There is some pretty atrocious stuff out there.
Some RVs have what *looks* like adequate and functional storage space and cabinets, but after you buy one you can find out they're awful. You can find dead spaces in cabinets with a blank panel covering them for ex. Or lack of a middle shelf in upper cabinets or an empty cavity that could have been used if it had a door (under bath vanities is an ex.) No place to hang a bulky jacket. Difficult to reach upper cabinets. TVs mounted in locations that are difficult to view very well.
Some RVs (thinking of trailers) have better constructed floors than others. Some have floor joists too far apart and too thin subfloor material and the floors will flex a lot when you walk back and forth. Friends of ours have one like that and you can see the floor move up and down when someone walks across it. You should be able to gently hop on a floor and it should feel reasonable solid.
Don't talk to dealer's sales guys. As long as you know what you want to ask, look for a factory rep. Sure they'll be biased, but will know way more than a dealer on tech. stuff.
There's a lot of things to look for and could go on for pages.
After a while, they just all seem to look the same though (sigh).... Also, it's not just the RV itself to consider, it's how good the factory or dealer support warranty work.
