Planning mileage was nine (9) miles/gallon with my '07 5.4V8 and 3.77 axles pulling my full body travel trailer. Towing mainly in Ohio, Pennsylvania, and New York. Perhaps more hilly than Michigan in some parts. Not enough towing yet with my exactly one year old '17 but it's a bit better and tows fine with the 3.31 axles.
Main factor appears to be wind resistance as towing my little C-Dory 16 cabin cruiser got about the same mileage despite being well less than half the weight.
Speed truly used more fuel. I tended to tow at the "truck speed limit" despite the trucks running well faster than it.

Speed also was a major factor towing without the trailer. Several dozen drives from Cleveland to Gettysburg (where my daughter
and money went to college) showed I could do it with no fuel stops if I ran the speed limit and in less time than if I ran the speed limit "plus 10" and had to refuel once. Still hard to drive slow, though.
My popup camper tucked nicely behind my Explorer and got better mileage.
"YMMV,"
-- Chuck