I think Jon put together a good list in the first response. Those are the things to look out for most are not deal breakers IF the price reflects any issues in that list.
I would add tire age. After about 6 or 7 years of the manufacture date stamped on the side wall (may be on the back side) the tires
have a greater risk of failure from the rubber degrading from the inside out.
Many don't drive camper enough to wear out the tread but the old rubber won't hold up to several hours of highway. Again not a deal breaker just something to look at and factor in. Might not want to hit the road for a few hundred mile trip back from buying the camper on old tires
Check that windows
function, as do inside and tail lights
, water pump and other electrical
systems. Old wiring that has been hacked at can be a pain to re-do or expensive if you pay to have it done. If they don't have a usable camper battery
bring one or plug into your tow vehicle.