Have it seal teched. That is where it is pressurized & checked for leaks. It will always find some potential areas but many would not have allowed water in anyway. Make sure there are no spongy areas especially close to cabinetry or inside cabinet floors. Look for any signs o f bulging on sidewalls outside. Check for cracks in frame underneath. If the roof is walkable, walk on top and check for soft spots. There is not much else you can do. Try to negotiate a warranty with the dealer. Ask him if you can talk to the previous owner if you promise not to discuss how much they were paid for it.
