First things first, check for water damage. If you're able to get up on the roof, look for signs of a cracked rubber roof. Check the caulking around windows, and check inside of cabinets, and compartments also for possible water damage. If its got a built in generator, run it under a normal load, like having the lights, TV, and A/C running. Check the heater, fridge, water pump, etc for proper operation. Visualize it being used by your family, and see how comfortable it'll be when say, the couch(s) are opened up into beds, and the kids are climbing the walls. Open up and look at the awning. If all passes and you're serious about purchasing, hook it up, and take it for a ride. Check the brakes, and lights. Unless the fresh, grey & blacks tanks are full (eww), you might not discover if any have a leak until its yours. Oh, and batteries. Yeah, they don't like to last very long, at least here in the desert SW. Good luck!