to FRF and Happy Camping.
The biggest thing to look for is any signs of leakage. Check the corners, around the slides, around the roof penetrations (skylight, vent, etc) for staining or soft spots. Search for the PDI list that's been posted here and elsewhere, print a copy and take it with you. Check things off as you go aver them. Don't be afraid to take your time, ask questions and go over things more than once.