Take a PDI sheet with you and go over everything. While you don't expect it to be new you do expect everything to work reasonably well.
Inspect the roof and caulking really well. If that's been taken care of the rest of the rig probably has too.
Inspect the tires. Not just the tread but the sidewalls for checking and the mfg date. The last 4 digits if the DOT number is the week and year of mfg. If they're more than 5 years old (which they probably are) plan on replacing them.
Check for delamination. Some units had a problem but it would surely have showed up by now if that unit was going to have an issue of it.
Look for tell-tale signs of leaking. That's the major thing I'd put on my list of "stay away froms".
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Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!
Yeah it's a pre-delivery inspection sheet. But instead of one the dealer uses it's one that you use to make sure you don't forget to look at everything on the unit. Here's a good PDI Sheet on the web. You can download it as a Word document and print it out.
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Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!