Most likely your limit will be your Tow Vehicle (TV) cargo capacity or pay load.
For example,
• My 5000 lb. dry weight Travel Trailer (TT)weights 6200 lbs. loaded for a trip. Tongue weight is about 700 lbs.
• My F-150 is rated to pull 9500 lbs. but it has a cargo capacity of 1411 lbs. That is what it was designed to carry. That leaves me with 711 lbs. of cargo.
• My DW and I plus our dog and the truck cap with our gear weight about 700 lbs.
• According to the CAT scales we are at or slightly over our TV weight limit when we tow.
As for gas mileage expect 9 to 10 mpg and you will not be too far off. I assume your TV has the towing package.
Also the only weight numbers that count are the ones you get from the scales.
Unless you are in a real hurry, rent a TT similar to what you think you want. It is a tad expensive but it answers a lot of questions very quickly; without the big cash outlay.
Twenty six foot 2010 Dutchmen Lite pulled with a 2011 EcoBoost F-150 4x4.
Just right for Grandpa, Grandma and the dog.