Check the DOT date of manufacture on the tires. If any tire is more than 5 years old, they should be replaced. Figure this into you price. Also check the coach and chassis batteries. Batteries are good for about 4 - 5 years. Check engine belts, hoses, filteeer, etc. Hopefully the previous owners kept good maintenance records. Also hopefully they kept the owner's manuals for all the goodies; stove, fridge, furnace, AC, voltage convertor, water heater,etc.
Check the roof seams carefully. Make sure there are no cracks in any of the caulking. Check for water damage, stains, soft spots in the walls and floors. If possible, run the generator and the air conditioner and water pump, and pretty much everything else that you can. Take it for a test drive.
Remember, this is a house on wheels. It will have everything and more that your house has, and it has to be able to move down the road. Good luck and let us know how things work out.