This question comes up on one site or another frequently, and the bottom line consensus as I read it, and from personal experiance, it's wind resistance by far that determines mileage. I've towed 5 and 10k pounds with the same truck, one ten feet hight the other 11. Mileage on the lighter one was just about 1 mpg better. And I'd attribute most of that to it being a foot or more shorter.
Also, my cousin bought a Honda Ridgeline and pulled a small, lightweight TT thousands of miles, coast to coast and back. Ten mpg. Maybe engines are just supposed to give you ten mpg if you pull. Who knows? Just my thought.
'11 Eagle 320RLDS
'02 F350 PSD Dually 4WD
DW's Ride, '13 Expedition