Look out for leaks...
Warping of the paneling, or mold is always a tell tale sign of leakage from the roof.
Tires...if the tires have dry rot, then you will have to replace. Maybe they will lower the price.
Ask to see that everything works or if it doesn't that they give you a list of things that are not working. How is the engine? How is the generator? Turn on the air conditioning, but also turn on the heater even in the summer to see if they are working.
Climb on the roof and take a look around. Holes? tears? Old caulking?
Of course look for leaks from the tanks as well. Ask to put some H2O in the fresh water tank to check for leaks.
Here is another view