if we were to buy a travel trailer for full-time living...
I'd start by finding out the towing capacity of your tow vehicle--that and your budget are the biggest limiting factors. Also, remember that the trailer will be carrying your stuff, and possibly water and sewage in the tanks, which has to be included in the weight.
A while back I had my eye on a 26-foot Nomad trailer from the 70's on craigslist: kitchenette, decent-sized table, and sofa in the front, bedroom with four bunks in the middle, and bathroom with small tub in the back. It seemed well-planned for carrying out the basics of cooking, eating, sleeping, and bathing, without any extras.
Condition: you need tires that are still good for the road, working brake and tail lights, a roof that doesn't leak, and probably a door that locks securely. Water and gas and electrical systems should work safely, if you're going to use those.
Best place to put one is on a little land of your own, if you won't be hassled for it, or in the yard of some congenial rural friends or family who will share their bathroom and laundry facilities with you.