Have you done a valve adjustment Cab?

On a stock never been fooled with diesel, there's not much mechanically that can affect fuel mileage.

Valve lash is one. Injector nozzle holes will erode bigger over time which will allow them to pass more fuel over the same pulse width but that usually takes several hundred thousand miles.

Another thing to check is your charge air cooler for leaks. Plug one end and make a fitting for the other end with a valve and a gauge to apply about 30 psi of shop air to it. Air it up and close the valve. If it loses more than 5 psi in 15 seconds, it needs a new one.

Kevin