F350,Crew cab,dually,4x4,4.10 gears, automatic:
I do not see how you hope to get great mileage with this combination.
F250, std cab, 2wd, 3.55 gears, srw, and manual transmission is going to give you the best mpg.
That being said, we did 21 mpg at 55-60 mph empty.
We still average 18.5 mpg at 70 mph 1800 rpm empty.
Our best towing mpg was going south on I-5 (fairly flat) and got 14.2 mpg at 60 mph.
We are not set up for maximum mpg because we have 4x4, s/cab, 3.73 gears and F350. But we did not buy our truck for maximum mpg, but to fill our needs in size and towing.
If I were you, I would not worry about mpg, or you will not be happy with your truck.