6 liter V8 Chevy Vortec question

Here's my experience. We have the 6.0 in a 4500 chassis under a 32' C. Loaded for travel, it crossed the scale at 13k pounds. In addition to that, our Jeep weighed 4660. We travel from the East coast to CO every Summer. On reasonably flat ground with no significant head wind, it will run at 2000rpm in 6th gear. On long grades, it will gear down and sometimes turn about 3200rpm. Running down the East coast to FL, it runs a steady 2000rpm and gets between 8 and 10 mpg, depending on wind.

Gary