Wind gets you more than anything. I get between 14-18 towing with a Jeep Liberty diesel.
Tell me about it. We just got back from a 4300 mile trip pulling a 180 with a 2010 F150 5.4. Went from Tennessee to Cheyenne, over to Salt Lake City Utah, down to North Rim Grand Canyon, Bryce Canyon, Mesa Verde, Durango, then back across Kansas and Missouri. Never thought it was possible to have a headwind both ways but I got lucky and got it! All flags flew straights towards me the whole way. Going across I-80 from Cheyenne to Salt Lake, they have the wind turbines all over the place. Guess why??? LOL Several times I was going down 6% slops in the mountain passes, had a head wind, and had to give it half throttle just to go 55 mph!! UNREAL...... Worst tank was 9.1, best was 13.3. Most all were in the mid 10's. If I ever make that trip again, it will be in a clockwise rotation.
Also going to look into putting a topper on the truck instead of the tonneu cover. I put one on an older Ford and it made a world of difference when pulling a 6x12 enclosed cargo trailer.