There are many things, some easy and some expensive, that you might not see. Water pump - $150, easy. Slides come off track - $thousands. Find out all you can, i.e. where they took it, how many miles, how may nights out, etc. Did they always plug in, or did they boondock? How is the chassis, frame, brakes, wheels and tires? Why exactly are they selling it? Some of these things, when cared for last 40 years. Some turn to junk in 5. After you've had this discussion, pay for and bring in an RV tech. Pay $200 or so for an on-site inspection, before you incur big problems. If you're buying from a dealer, new or used, get a warranty. But if you're buying from an individual, spend the money on an inspection. Good luck!