There should be lots of used 5vers near you. Check the ceiling and walls for any discoloration, that often means a water leak and they can be tough and expensive to fix. Get up on the roof and check the rubberized coating and especially around any roof penetrations: vent fans and so on. Check for any soft spots.
A couple of things we were told to check by various people: look up at the area over the basement, if it's pressboard or chipboard then I would down rate it especially for long term use.
Also one dealer told us to look at the wiring in back of the breaker box, if it's neat and organized that's a good sign. A disorganized mess: bad sign.
There are all sorts of check lists online, search for RV PDI list or predelivery list. And take a good one with you. They will have a lot of items for you to look at, we found a couple items when we did ours, one vent fan didn't work for one thing.
x2 on the above get everything running. Roll down the awning and so on.