I have a 2013 F350 dually with 3.73 gears, pulling a 14,000# GVW 5th wheel (rounded nose).
I've traveled about 4,000 miles so far, all east of the Mississippi. I try to run between 58-60 (I'm retired) and budget for 10 mpg. So far, my best one day average was 12.3 and worst was barely 10 (pushing into a headwind all day). Just about all the other daily averages were between 11.5 and 12.0.
Truck has about 9,300 miles on it now. My gut feeling is that the mileage seems to be getting slightly better as more miles are put on the truck. If it matters, non-towing mileage at 60 is in the 20-22 mpg range. For a truck that makes 400 HP and 800 pounds of torque, I'm real happy with these numbers.
Tim