What to look for when buying used TT

Well a quick update, drove 3hrs to see this unit.. And.... It's still parked 3hrs away.. It was not a total mess, but had just enough wrong for me to walk away.. Way over called roof and roof vents, some cracked windows, unit smelled like propane after testing the stove and shutting off the valve (and triple checking them) for 15 min afterwords. All the equipment was original, it all worked, but looked there age.. We just were not I love with it so we walked.

Thanks for the advice, the search goes on.