I have the Cummins ISB 6.7 in my Fleetwood Excursion, 31,000 pounds, 40,000 miles so far.
Last 2 trips totaled 1100 miles, half of the miles we were towing my old jeep YJ. I averaged 10.2 MPG, love the engine.
Earl & Sharon, 2008 Fleetwood Expedition, ISB 6.7
1995 Jeep YJ/2013 Chevy Sonic