Although I will agree that driving technique can account for up to 20% of a vehicle's fuel mileage, sometimes more.
Beyond that a religious maintenance program also helps, change the oil and filter, the air filter, the spark plugs, the list goes on...
Just one underinflated tire can cost 5-10% in fuel economy, so keeping up with this as best as one can does make a difference.
But beyond that with some outfits it's all in the engine and the weight, my '95 3/4 ton club cab with a 5.9L V8 and a 6x12 trailer
weighs 6 thousand pounds empty, I can almost see 12mpg on a very good day, under 10 when it's not so good.
There ain't NOTHING I can do about it, not even "driving with load" to a fault helps but so much.
My '86 D-250 used to get 14mpg all the time, it used to drive me nuts.
I mean I have gone so far as to study the aerodynamics of the truck to figure out what modifications I can make
I kid you not, I spent WEEKS fooling with that
In the end I just had to face the music, some trucks don't do as well as others.