Your truck works though. I imagine you do alot of short drives, idle time while talking to customers, run to whereever to pick up the needed material etc.
You'll get different mileage from a daily driver to a working truck.
If I drove the same route every day, the same 20 miles with 1 stop sign, and 2 red lights, I would get different mileage than driving 20 miles in Chicago.
When you plow a parking lot- it takes more fuel to get you moving than it does to keep you moving. If your moving 4" of snow it takes less effort than moving 6 or 8". Same RPM but it may take more throttle input to move you. This goes into converter slipping etc.