speed kills
MPG
The aerodynamics of our trucks is such that the power required hold high speeds increases exponentially. It takes nearly twice as much power to go 75 mph as it does to go 45 mph. So drive slow and keep your RPMs low and you'll have excellent MPG... considering the size of the vehicle.
Unfortunately, that's crappy advice because no one likes to drive that way. If you are serious about better mileage, try limiting your top speed to 65 mph for tank full. If you do at least 50% freeway cruising, you'll see a difference.
As for parts, the Edge EZ & RV 275 injectors netted me about 1 MPG increase, cost $550 (used). A toneau cover netted 1 MPG, cost $400 (new). $950 still buys a lot of diesel in this country. Change driving habits - FREE. Properly inflated tires - FREE. You decide how you want to spend your money. Reducing weight and drag helps more than increasing power.
BTW, I bought the box and injectors for performance improvement, pretty good value for that price. I bought the bed cover for cargo protection from weather and prying eyes. The MPG gains are a bonus in my reasoning.
P. S. My average commuting MPG is 20. 11 (50% freeway), best tank, 450 miles, all freeway, top speed 65 mpg - 21. 7 MPG, average towing my 5th wheel (about 16. 9K GCW) ~12 mpg, average lifetime MPG, towing and all - 17. 09.
Enjoy the ride,
Neil