With any used RV, make sure the unit is checked for water intrusion. Have your checker carefully examine the area of the cab-over bed, all corners, and around the windows for evidence of water, moisture, or "softness" which indicates rot. Check all windows, the inside roof area around the air conditioner, and any other items that poke through to the outside roof. As noted by others, have them check operation of all appliances, generator (if equipped), and get your checkers to find out the Occupant and Cargo Carrying Capacity of the unit. Also, determine the age of the tires (check the date codes on the tires). They will need to be replaced if six years old or more, no matter how good the tread looks. Good luck for a successful purchase.
