Fred,
There are many variables that influence fuel economy, and even though the C 12 is a very fuel efficient engine, I do not think that you should expect a consistent 10 MPG. From my experience, your coach normally runs in the 8 to 9 MPG range.
I have driven two different C 12 equipped coaches towing Jeep Grand Cherokees over the last 13 years, a 38,000 plus pound 2000 Marquis (8 to 8.5 MPG) and a 50,000 plus pound 2002 Solitaire (7.5 to 8 MPG), and in about 180,000 miles, I have never seen a consistent MPG over 8.5 regardless of the speed. I have seen 10 MPG on trips with a consistent tail wind, especially if I was dropping in elevation, but I have seen 5 to 6 MPG with a head wind or when gaining in elevation.
Gerald