The trailer is 18 years old. If it's survived this long, how many factory defects could it possibly have? It *HAS* to be a good quality rig, by virtue of having survived this long.
Just look out for the usual stuff. Obvious signs of delamination, leaking, structural damage. Make sure the appliances work and the plumbing is intact and leak-free.
Good luck finding an old-style 1991 GM dually. There weren't a lot of them made to begin with, and they're 23 years old now. There aren't many left, let alone many that are in good enough shape to do long-distance trips without a LOT of work.
Actually, I do know where an 1990 or 1991 GMC crew cab dually R/V series is fairly close by. It was on the local craigslist. Used to be a dump truck.
The caveat is, it has no engine or transmission or bed, and needs a LOT of work to become roadworthy. Probably not what you're looking for.
Putting 10-ply tires on half ton trucks since aught-four.