MPG 2017 3500s pulling 22,800

We have a 2012 Kenworth. It has a 15 liter Cummins coupled to an 18 speed and 3 drive axles with 4.10 gear ratios. The truck is not really set up for highway use and is not capable of hauling much weight legally. We seldom take it on the road. It has a top speed of only 68 mph @ about 1600 rpm. The empty weight of the truck is very close to 33000 lbs. It has a 20,000 lb front axle and 69,000 lb rears. On Alberta highways it requires a permit to be able to load the truck to 66800 lbs. We use the truck mainly off road and load it to about 95,000 lbs. I took it down the highway empty the other day and measured it's fuel economy ..... 9.6 mpg (imperial) or 8 mpg US.