Don't drive it in 4x4 on the highway.
Keep a logbook, don't trust the MPG display.
Buy fuel from a different source and compare. Winter diesel and summer diesel produce much different MPG.
You say nothing about how it was being driven to produce 10 MPG. Depends on what you are towing but that might be good MPG for towing.
As a rule of thumb the most efficient HP is produced at 2,000 RPM. That doesn't say 2,000 RPM is where the best MPG can be hand. It means that is where your HP is the cheapest. Accelerate at about 2,000 RPM for best economy. Being too light footed may be producing results contrary to what you were expecting (but not 10 MPG worth).
Cruise control at 60 MPH today with two dirtbikes in back I saw 18.8 MPG on the fool computer. 15 miles of traffic lights sucked it down to 16.5.
Pulling a friend's toy hauler at 70 MPH produced 7.5 to 8.0 MPG.