Two things here:
1) The fuel usage meters aren't very accurate, and are calibrated on the optimistic side. I'm rather curious about the bully dog setup... is that a piggy back? Know how they work, on a signal level? And how those signals are also used to calculate estimated fuel usage?
2) Most importantly, varied between 40-70, probably a 55-60 average, and I'm guessing in the UP... which means flat, straight, and very few stops.
I can completely see that mileage for that situation, unloaded truck (sled does nothing), those driving conditions.
For example, no matter the vehicle, when I visit my parents (180 mile round trip, completley flat very few stops, 55mph two lane roads, in the thumb), our vehicles get 20-30% higher mileage than our normal mileage. Last trip there with my V10 dodge, I got 11.7mpg towing an 8500lb load (large tractor with loader, forks, other stuff), that be 8-9 mpg down here.