In my opinion the biggest problem to look for is any sign of water damage. Get on the roof and inspect the condition of the roof and the caulking around anything that sticks through the roof. Check the date code on the tires. Likely the original tires are on the trailer unless there has been a problem. If the tires are over 3-4 years old plan on replacing them. Have the seller demonstrate that all of the appliances work (refrigerator, water heater, stove, microwave, water pump, power converter, etc.). Make sure the slide(s) work properly. Recheck for water damage and walk away if you find some. These a few quick thoughts on the topic.
2014 Eagle Premier 361REQS
2013 Ford F350 Crew w/ 6.7 diesel