I get 11-13 mpg with my '15 F250. The bigger factors are wind, terrain, and traffic. Having come from a '13 Sierra 1500, diesel is definitely the way to go. I will never tow with a gas motor again. My trailer is right around 11,500 loaded and my truck tows is easily. Like Ultraboy said, let the torque work for you. On the interstate, I set the cruise at 70 and except for bigger hills, the truck never downshifts. The GMC was always downshifting, even with a 16' trailer that weighed less than half of my current TH. I'm very happy with the truck overall, very happy with the towing.
2015 Work n Play 30WLA
2015 Ford F-250 Lariat diesel, crew cab 4x4