I don't have a Detroit, but my Cummins ISX15 gets 7 mpg running at 70-72 mph as calculated by several tanks filling and mileage calculated by hand. I don't really notice much mpg difference if towing trailer or toad vs MH alone. I am about the same weight and size as you are.
I could probably get better mpg if I would go slower, but time is of the importance to me. I want to get to the location. Plus in general I hate being stuck in slow lane.
2005 Kenworth Showhauler 45 ft total length
twin screw, 450 hp ISX, 10 speed Ultrashift