Right after I got my truck, I took a 2000 mile trip towing my 6500 lbs (dry) travel trailer. trip average was 10. 6 mpg including a couple of hundred miles in 4 hi due to snow. I had done the identical trip with my '96 ford powerstroke the year before with the same trailer. The 215 hp powerstroke averaged 10. 3 mpg, but keep in mind that it was a 5 spd. On my recent trip to Baja with my camper, I was getting between 12 - 13 mpg on the highway, running at about 70 mph. Total GVW was around 10,500 lbs, but more wind resistance than my trailer. I believe that the truck is getting 1 -2 mpg better now than before the break-in process was complete. My friends 7. 4l vortec was getting 6. 7 mpg with the camper on. All things being equal, I believe that a Cummins will get 1-2 mpg better than a Powerstroke, but a 350 hp Cummins won't get the same mileage as a 215 hp Powerstroke. I had a friend with a 160 hp Cummins and he got 23-24 mpg on the highway, empty.
Dave