OP here with a new setup.
2002 F350 SRW, 7.3l power stroke, auto trans, crew cab, long bed, 4X4 & a 2007 Snow River 9'6" with a slide out.
Just got back from a trip to Baja. Of the 2600 miles I traveled in the states (& kept mileage records for) I averaged 10.4mpg. Best was 11.8, worst was 9.3...