The reason I referred to it as the “elusive” 10 mph was because of my two trucks.
In 2012 I rented a 25b ft. 4800 # Edge and pull it to the Ozarks with a 200 hp. V6. I got 10 mph of the flat with a max speed of 60 to 65 mph. in the Ozark hills it was all I could do to pull 50 – 55 mph the mileage was at best 9 mpg.
Last December I bought the afore mentioned 2011 f 150 with E-boost. with about 350 hp. I expect improved performance and gas mileage. Well as far a performance is concerned the turbo charger was a real treat plenty of power. However, it was the same 9 mph in the mountain and 10 mph on the flats.
Judging from the reports on this forum if you get 9 to 10 mph you are about normal no matter what the engine, provided you have a reasonable TT to TV match.
The lucky ones seem to squeak and extra mile or two.