Water damage is the main problem. Look for any outside or inside panels that have a wave/bump in them.
Especially look around front windows since those seals are weakened by he force of the wind.
There is a lot to test to make sure it works. Electrical, propane, plumbing. See if the seller with take 1-2 hours with you and show you that everything works.
Expensive repairs:
Anything related to the truck part itself. Same as buying any truck.
Refrigerator has to be tested on 120v plugged in and also on propane.
Tires should be replaced after 7 years. A lot of people mostly stay parked so dangerous tires are often on older models.