That's great gas mileage for a big ride like that.
A lot of the newer big trucks get decent mileage.
Depends on how you drive it.
There are guys on the F150 forums that get 12-14 mpg, because they drive like idiots.
My dad has a newer Chevy Silverado that gets 22 mpg on the highway. It shuts off 4 cylinders when you are at cruising speeds.
I drive like an old man, so that helps too.