I own a 2012 F-250 CC/FX4/Lariat 6.2L pulling a 2015 Jayco 28BHBE (33 ft travel trailer_. Average weight towed between 7500 - 8000 lbs. Towing gas mileage is 8.5 mpg if I stay at 70 mph or below. Anything over 70 I start dipping into the 7 mpg range. The truck has plenty of power to run the high speeds but it will cost you in gas. Not towing I have seen everything between 12.4 and 17.1 mpg. Again, staying at 70 mph or below will significantly increase your mileage.
This truck was an upgrade from my 2012 Tundra Crew Max 4.6L which did not have enough power or suspension to safely tow the Jayco. With the Tundra, it was a struggle to maintain 65 mph and the handling was dangerously questionable at high speeds, even with the weight distribution hitch. Truck was supposively rated for 8100 lbs towing.
I read somewhere on the forum that you want to stay away from operating within the final 20% of your max weight (towing/GVWR). After trying that out for a few thousand miles on the Tundra, I am a firm believer in that concept.
If I had towed more than once a month, the Ford 6.7L diesel would have been a better choice, power wise. However, my needs and finances were in favor of the gas. The diesel is unquestionably better in HP and torque, but the 6.2L will cost you far less in the long run. I also use my truck as a daily driver and my wife loves riding in it.