My numbers From two different trucks. One a 2011, the other a 2015. Hauling a 2006 Okanagan 117DBL 6K pound camper
2011 4X4 3.73 gears Maintaining 70 mph, 9.695 mpg over 1039 miles.
2015 4X4 4.30 gears maintaining 70 mph, 10.237 mpg over 1101 miles.
Both trucks 6.7 PSD F-350 DRW. The 2015 has the high capacity trailer towing package.
My thoughts on the lower mileage for the 2011. Had one tank that was 10% bio diesel and it had the worst mileage which dropped the overall mileage of the trip.
The 2015 trip also had a very high tail wind most of the trip.