There are way too many factors involved in fuel economy to be super consistent and applied across the board. For instance my 10.5 mpg over the 1800 mile trip was driving the interstate speed limit, 70 to 75mph mostly, my GCVW was around 18,000lbs. Terrain on this trip was from the Boston mountains in NW Arkansas, down into the Mississippi River Valley, across the long rolling hills of Tennessee and at the edges of the Smokies, ending in Newport, TN. Now smaller trips that I have taken to Texas and what I consider flat terrain, I have approached 13-14 hauling the same load at the same speeds.
2005 Forest River Sandpiper 285RL Towed by 2000 Ford F350 SRW 7.3 Diesel.