What I do; I calculate DPM (Dollars per mile), when planning a trip. For example; What's the approximate worst mileage I'll get on a trip? 7 MPG. OK, if I'm going to go 2,000 miles, I figure 2,000 divided by 7 equals 285. Times 3.50 per gallon equals $997. So I budget $1,000 for gas before I go. If I spend some of that time cruising flat highway, and get 10 MPG on those stretches, I come out ahead. I usually do track my mileage just to keep myself informed, but I don't sweat it because I budgeted for it.
OP, my previous truck was a 2003 f150, same engine as yours. The best I could do pulling my current trailer (7500 lbs) was 9 MPG, and it struggled. My new truck gets about 10 or 11 MPG towing on the flat highway, but it sure pulls the trailer easier. Best move I ever made buying it.