Robert I have had pop-up's, truck campers and pull behinds. When buying used the numero uno thing to look for is WATER DAMAGE. Inmho no camper built can stand up to being outside 24/7(I know I know they are supposed to be designed as outdoor products) and sooner or later they all leak. When they do the damage can be minor if caught right away and the issue fixed. If not, the damage will be extensive.
So, when looking, take a really powerful flashlight, really small screwdriver or ice pick, and a calibrated schnoz. Always crawl up on the roof and inspect it for soft spots and visible damage. Then look closely under the camper at the area where the floors met the wall and the entire ceiling. If you see something that looks soft, poke it lightly with the pick. If it's rotten you will know it right away as the pick disappears into it. Inside look at where the floor meets the wall, under every window & look under every compartment or seat that is removable and SNIFF for mold/mildew. Check all the plumbing fittings under the sink/shower for leakage. Pay attention to the floor inside for soft spots.
If you find or smell any of these things walk away and don't look back. Water damage repairs can cost as much as a used camper is worth if you have it professionally repaired. Remember, water damage is like a floating ice berg....you are only seeing a small portion of it. I have tackled two campers that had what I would call minor damage, both took many days each to tear out and re-build so I am done doing that kind of work...it sucks. Anything other than water damage is usually a pretty straight forward repair.
When you do find that nice pristine camper really think hard about putting it in a garage or portable shelter. If you do this it will last forever as long as you keep the mice out....all mice must die:devil: