As I have bought older trailers, not these exact models, my .02:
1. Do a really good walk around with a flashlight, yes even in daylight. Shine it up on the walls, you will see if there are problems.
2. Walk every inch of it inside, if you have a weak floor you will feel it.
3. Have the seller put water in it. Turn the water pump on, you should hear it run for a few seconds, then turn itself off. Once it turns off, leave the switch on, turn on a faucet just long enough you hear the pump kick on again, then turn the faucet off and when the pump shuts off again, turn the switch for the water pump off. Find something else to do and time it for about 6 or 7 minutes, then turn the pump switch back on. I SHOULD NOT come back on, if it does, it has a plumbing leak somewhere. Plumbing leaks can cost you a lot to fix.
4. If it has a walk on roof, get up there and see what the caulking looks like. Does it appear a year or two old at the most or does it look like it has been there since it left the factory. Will give you an idea if the current seller has done regular maintenance to it.
5. Ask the seller the last time the bearings were done on the wheels.
I could go on, but IMHO those are the big ticket items to look for. Remember it is an older trailer, it isn't going to be perfect.
2015 32 Foot Rockwood 8289WS 5th Wheel
2017 GMC 3/4 Ton TV