OP - Your plan sounds more thought-out and feasible than some I've seen here (compared to a family of 4 wanting to winter in upstate NY in a fiver). So I'll start with a couple of suggestions:
First find a mobile RV tech that you can hire to thoroughly inspect any trailer before you sign any purchase paperwork. You want this on your dime, so he/she is accountable to you.
Second, call 1 or 2 places where you think you'll first be camping for a while. Ask them if they have a local outfit that rents 50-gal or 100-gal propane tanks and get that referral. Call them and get pricing and guestimates on how often you'll need to have them filled.
I'd also recommend creating a generous time and money budget for the first year of outfitting/customizing whatever TT you buy. Our 18-month old (when we bought it) unit swallowed somewhere between $1,000 to $2,000 in tools, supplies, and projects our first year.
