Could be hard to get an accurate answer since most of the lower HP motors like a 300 would probably be hooked up to a automatic trannie versus a manual or manual automatic that the bigger motors have. It would be hard to compare apples to apples.
I will say that my Cummins M11 350 hp & 1200 ft/tq with the original Fuller 10 speed averaged 11.5 mpg. That motor had 550,000 miles on it and well broke in. When I swapped the trannie to the Super 10 speed it killed my MPG & power quiet a bit. That might have returned as the trannie got some miles on, but by then I had sold it. My weights are probably pretty close between my new and old motorhome.
I hope to have my engine monitoring system later this week and I will have lots of info to give you once that is installed for my 300 hp setup now. If I remember I was getting approx 8 mpg when the engine was new and I think if I remember it creped up to 9 or 9.5 during one of my later trips and that was at 280 hp. It will be curious to see what the increase in horsepower does to my MPG.
2012 Showhauler 28'6" Motorhome on a Columbia w/ 450 Mercedes.