2013 Ford F-350 DRW 4x4 crew cab long bed, 3.73 gears, carrying 14,350 total with Northstar Igloo in the bed. 
I averaged 11.7 mpg with the 6.7 diesel during a 11,000 mile trip to Alaska. 
I tend to push the speed- so more 70mph than 60. In northern Utah where the highway speed limit is 85 I set the cruise control at 95 and it the short term mileage was still above 11. 
I added a 14x7 dual axle trailer pulling maybe 5000 lb. and the mileage over 600 miles was 11.4mpg. 
Running empty around town about 13mpg
highway empty 15mpg.