I have a 2016 Ford F150 with the 3.5L Eco-Boost engine. I average 20-21mpg around town, rural community. 24 on the interstate unloaded. Towing my 7,000 5th wheel I average between 12-13mpg at 60mph. Jump up to 65mph and it drops to 10-12mpg. My truck is a Supercab and holds 5 comfortable, grandkids in the back. Towing my 2,300 boat I average 17-19mpg at 60mph. Triple towing the camper and boat I average 8-10mpg. Your mileage may vary.
Before this F150 I had a Chevy Traverse with towing package, up 5,000 pounds. It had 4 Captain chairs and a rear bench. Around tow I average 18-20mpg. Interstate 20-21mpg. Towing the 2,300 boat 14-15mpg at 60mph.
There are trade offs. If you buy a vehicle with a V8, you get average gas mileage all the time and slightly better mileage towing compared to 6 cylinders. Get a 6 banger and you will get better than average gas mileage all the time and less than average while towing.