Rear sag and bad axles. Each of these problems can cost you around $2k to fix, so it can be a big deal.
Do a search here on both sag and axles. If you can fit even a pencil between the frame and the body in back, it's sagged.
Next down the list is the refrigerator. Replacement is about a grand, so make sure it works on both gas and electric. If, on the other hand, it works on one but not the other, it's probably just a broken prart, and maybe on ly half that much to repair.
Air conditioner. $500 or so if busted.
Check the floor for soft spots.
Water leaks. Does it small damp?
Are the propane tanks updated? Are the tires good? Do all the lights and stuff work?
Is the awning fabric ok?