I have a Sterling Acterra with the 926 engine at 300hp/860tq and the 6sp auto. It scales 20120 with full fuel and all my stuff and me. Straight off the dealer lot and running about 62mph it got 9.95. Next fill and similar running was 10.15 and the fill after that, still running empty and around 62mph was 10.25 mpg. I'm waiting to be loaded now and will see what it does with some weight in the box. I'd guess the lower hp would do some better on mpg, at least empty and under light loads.
I calculated my spreadsheet figuring 8.5 mpg avg at $2.25 gal diesel. I expect to do at least that well and probably a little better on mpg and at least for a few more weeks a little better on price also. I can't see anything over 12mpg being remotely reasonable though. I'd use around 9 or at most 10 in figuring expenses. Good luck.