Fuel mileage help

Hello everyone. I am trying to get some fuel mileage advice. I own five 2014 trucks. Two t800's and 3 western star 4900's. All are 500 horse cummins. We haul logs on short runs. Lots of stop and go, hills, off road driving, etc. we also are hauling an average of 85,000 lbs. my trucks are average anywhere from 5 mpg to 5.5 mpg. I was hoping to be able to get above 6. Am I dreaming, or is it possible? Have tried the progressive shifting in the trucks and we'll see if that works. Wondering if I need to switch power next time. Any thoughts/ insight would be greatly appreciated!