It's not that much different than buying a used car.  Check over the mechanicals to see that everything works, maybe take it to a nearby dealer to do an inspection, although you'll obviously want to work something out with that dealer in advance. Look at the cosmetic condition of the camper.  If it's in good condition, there's probably a reasonable bet that they took good care of it, but it's not a guarantee.  Look for evidence of leaks.  Climb under the camper and look to see what condition everything is in. 

Here's a site that's a little more specific:
http://www.title-3.com/BuyUsed.htm

I'm sure plenty of others will chime in with other helpful suggestions.

One thing I learned, people on the various forums tend to take better care of their toys.  Call it pride of ownership.

Fingers crossed for you. Good luck.