My 2012 F-350 king ranch long bed crew cab dually gets, on average 10.5 to 11.5 (hand calculated) pulling my 41' 5er, weighing 19,000lb. loaded. With no camper, about 16 city, 19 highway. One thing to consider is the driver. If you haven't driven a diesel much, your mileage will be lower. You have got to learn to keep your foot out of it, and let the torque work for you. Applies accross the board, but maybe more so with drivers that do not own the truck.
One of the first things I did, was to put a Titan 65 gallon tank under the bed. It replaces the stock tank. Gives you extra time to find a truck stop when towing long distance.
2014 Vengeance VGF39R12
2012 F-350 DRW King Ranch