Man, I sure wish we could get mpg like that with our F250 and am jealous...
Our F250 is 10 years newer with 6.8l V10, 4.10 gears and tow a 7K lb TT and always use tow/haul. We usually get just over 8 mpg and and rarely get 9+ mpg unless it's a long, long downhill stretch with a tailwind. Doesn't matter if I go light on the throttle or slower or on long flat runs, it's still in the 8 mpg range. Has been down 6 something mpg up steep mountain runs. All the mountains on the west coast don't help but sure add to the fun!
Our old 2000 F150 with a wimpy 4.2l V6 non-turbo engine pulling a 20' TT used to get around 6 mpg on average. So now with a much heavier TT & TV, we gained about 2 mpg.
Mpg is what it is and you have to forget about it and enjoy the towing experience! :)