I'm new at this myself but here are the things I was told to inspect (besides the obvious):
1. Tires. Check the tread but also the side for cracking. Tires exposed to the sun typically need to be replaced every 5 to 7 years even if they look fine. The DOT code on the side walls should tell you how old they are.
2. Check for water leaks. Look around the ceiling, windows and in cabinets for any sign.
3. Check for roof. Make sure everything is sealed and no visible damage.
4. Do all the appliances work?
5. Is there an owner's manual and someone to give you a walk-thru if you decide to buy it. If you're new to this (like I was/am), you will have a million questions about how everything works. The books I mentioned are good, as well as people on this forum. The internet is also a great resource.
Good luck and I hope it works out for you.
2007 Forest River Sunseeker 2900 Ford E-450
Mod1: LED lighting throughout.
Mod2: Replaced C715 cargo locks