Last trip from Griffin GA to Moab UT my rig averaged 13.6 MPG. See signature below. Tow speed ( flat lands
)was 65 mph most of the time. Truck was loaded some. TM carried clothes, linens, a few food staples. Only water was in the hot water tank. The truck is powered by a 5.7 liter V8. Pretty standard fair. It was not built for speed.
I have no two wheel drive data to compare it with.