If it has slides, check to make sure they never leaked and damaged the floor.
As with any used RV, check the floor all over for signs of softness, especially by the door.
Check under all of the counters and cabinets, and in the storage areas, for water damage.
Look at the tires to make sure they are wearing evenly. Any odd wear patterns might indicate axle problems.
Get up on the roof and check all of the caulked seams. If there is any spot that looks like it could have leaked, run away, run away fast!
And, of course, make sure all of the appliances work.