Hey Vernon - welcome to the Jayco Forum! Glad you found us.
One of the biggest things to look for are water leaks / roof condition. Take a good look all around at the ceiling, walls, inside compartments, etc and look for evidence of any damage or cosmetic repairs. Also - are the tires original? If so, they are probably due to be replaced so you can factor that into your haggling. How about the appliances? Verify they all work.
Here is a good thread that might help. It has several checklists for new trailers, but no reason why you can't use them for used. Obviously everything will not be in perfect shape, but it is a starting point.