My 08 6.4l F-250 diesel gets close to 800 miles on tank of fuel when not towing and over 650 miles when towing. Of course I have a 38 gallon tank and this is what I tow with it.

It routinely returns 23-24 mpg unladen on the highway, but when the regen cycle kicks in the MPG drops like a stone. The around town mileage is pretty bad too, depending on traffic. I bought it to tow a 30ft TT when I retire to a full-time mobile lifestyle in a few years. I expect to get around 12 mpg towing my FT TT, but I tend to drive very conservatively.
The 2 little 125cc motorbikes in the bed get around 100 mpg. Of course their tow rating is pretty low. ;)
Chip