The facts from the manufacturers is that they have been adding gears to the transmissions so that they can pick the best rpm for the load on the engine to obtain better fuel economy. I realize the 6.7 diesel and any other engine have their own BSFC graphs and that to get best efficiency the engine would run in a sweet spot. I can see from the graph that you posted that the 6.7 engine is most efficient when running from about 1500 rpm and producing 130 hp to something like 225 hp at 1600 rpm and at1800 rpm it is most efficient producing between 150 and 225 hp.
The reason I’m asking you for your actual measured mileage is because I would like to see where your truck fits on that graph when towing your rv.
