From all I've found checking Diesel Stop and other Ford forums you're likely to see mpg in the 10-13mpg loaded and 13-15mpg unloaded. Several Earthroamer owners on this forum have reported their mileage in the 12-13mpg range.
Several issues will impact the mileage, including your choice of wheelbase, axle ratio (4.3 vs. 4.88), transmission (auto vs. manual), etc. If you add any lift and/or offroad tires you'll also impact mileage. The other big factors will be height of camper, overall weight of the rig, and your own driving habits.
Hope this helps. Please post specifics about your proposed build.
Bob