Water leaks kill RV's. If you are able get on the roof and look at all the seams. If there are a bunch of untreated cracks you could have a problem. Next look inside at the roof for water stains. Look in the corners even inside the cabinets. And finally check the floor carefully for stains or signs of leaks. In any suspect spots push down with your thumb or the tip if a screwdriver and see if it's soft. Cracked caulking on the roof is a $10 tube of Dicor. If you find any soft spots it's your choice whether or not to buy. But personally I would run.