It's not so much the weight as it's the wind resistance you have. Weight comes in to play on the grades and such of course, but rolling down the level road, it's that big thing blocking the wind that matters.
If you get 10 mpg's towing, consider that about right and that's what it's gonna be. Don't matter if you get 25 mpg running empty on the hwy... Hitch that brick up and watch the mpg's drop..
Try taking a walk and put a little parachute on your back... How much harder is it gonna be to walk compared to not having that strapped to your back?
Add a head wind and all bets are off..
Good luck!
Mitch
2013 F150 XLT 4x4 SuperCab Max Tow Egoboost 3.73 gears #7700 GVWR #1920 payload. 2019 Rockwood Mini Lite 2511S.