At 55 mph, my 2005 F150 (less than 77k miles) towing my 20' Nash I am blessed with 10-12 mpg; at 75 mph I am blessed with 8-9 mpg. And both the trailer and truck bed are fully loaded (water, food, beer, firewood, gear, tools, etc). When trailer is empty, add 2-3 mpg.
Highway mileage--my F150 routinely blesses me with 15-17 mpg (and that's with regular gas that's ethanol blended). Stop and go city driving I get 11-12 mpg.
IMO, sometimes that extra 10 mph (75 mph) is not financially worth it, even with an oversized gas tank.