We have an F350 with the 6.7L Powerstroke, 3.73 diff. When the truck was stock we got 18-20 mpg (unloaded) on the highway 65-75 mph. With some changes including 34" tall AT tires (stock was 32"), an ARB bumper (not the best aerodynamics), and a flatbed which is a little wider than the cab, our unloaded mileage on the highway dropped to the 16-18 mpg range. (Odometer is corrected for taller tires, this is the "real" mpg").
With an XP V1 with all our stuff and about 70 gallons water (fresh and grey combined) we recently averaged 14 mpg eastbound through Nevada (US50) and Utah (US50 to I-70), at speeds 65 to 70 mph. Our loaded camper weight should be about double of a loaded FWC. Figuring out your fuel requirements in advance for off-road travel is tricky, I don't really have a number for that yet for this rig. Maybe 11 mpg.
FYI, for anyone who also tows, I recently pulled a Kubota tractor on a goose neck equipment trailer about 400 miles, the loaded trailer would have been about 12,000 lbs (did not scale it), and the mileage calculated by the truck was just over 10 mpg. That was mostly on the highway.
