Hey Bob...
Like a used car...best to get a used RV thoroughly inspected. May cost you a few hundred dollars up front, but will be worth it. Also depends on what you are going to do with it. Tow a few thousand miles a year or sit as a hunting cabin.
Check for leaks, all the appliances work, some say tires older than 6-7 years need to be replaced, brakes, bearings, etc...