My wife and I just did 3500 miles pulling a 197 Ultralight with a 2013 Ecoboost Super crew. At 90 kph, we would average around 13-14 miles per gallon. At 100 kph, down to 10-11, so, as others have said, drag is the deciding factor. If you can keep your speed down, your mileage will fare better. I stayed in tow/haul and 5th most of the time. At 90 kph, it ran under 2000 rpm. Didn't seem to be laboring in 5th.
We had all kinds of great big 5th wheel jobs go by us like we were backing up. I can imagine they were getting great mileage.