We have 2003 24ft E350 powered rig. We drive mostly interstates to national parks. Our last trip from Seattle to Yellowstone, usually traveling at the speed limit (70-73mph) we got about 9 mpgs.
Last year we had a 3000 miles round trip to the desert southeast fighting head winds both directions also traveling in the low 70mph range at about 7.5 mpgs. During the days with really strong head winds we spent hours at a time in 3rd gear (out of 4) at 70mph - about 6.5 mpgs on those tanks of gas.