There are some basic safety items you should make sure are in place before you buy.
1. Make sure there is no rust on the frame that would weaken the structure
2. What kind of shape are the tires in? If they are old, worn badly or have sidewall cracks, you'll probably have to invest in a new set before towing.
3. Make sure the hitch and all the mechanisms that go with it are in sound shape.
4. Can you have someone pull the wheels and check the brakes for wear and proper function?
5. Do the holding tanks leak? Do the holding tank valves work properly?
6. Check the roof and interior ceiling for any sign of leaks.
7. Verify that the refrigerator is in working order.
8. Check the furnace, A/C and Stove to make sure they work.
9. Does the previous owner have a maintenance history?
10. Fill the water tank, and turn on the pump. Does it come up to pressure? Do all the faucets work?
11. Pull out the awning and check for excessive wear on the fabric and make sure that the hardware moves freely and is in sound shape.
12. Make sure there are no leaks in the propane system.
13. What kind of shape are the batteries in?
14. Make sure the smoke alarm, propane detector and carbon monoxide (if equipped) detectors are working.
I'm sure others will have items that you should check, but those are the basics. If something needs repair or replacement, it will be a good negotiating point.