If you are buying from a private owner, ask what kind of maintenance work they've done to the fifth wheel. Make sure they have kept up on re-sealing joints and windows (anywhere water could get in). Ask if they've kept it covered when not in use and if they've washed it on a regular basis. It's also a plus if they had any coating applied to it assuming they bought it new.
Other than that look for any water damage, peeling paint or delamination. Look for any signs of infestation (material chewed away by the floors or corners of cabinets, beds, seats, etc).
Hope that helps