Drop back from 65 to 62 mph.
Use this calculator, you will see that giving up a few minutes can give you a couple mpg.
https://www.omnicalculator.com/everyday-life/speed
Another suggestion is to pump up your tires. I went from 32 psi to 42 psi on my truck and this helped a lot. Propably my best mod.
Also, is there a factory chin spoiler made for your truck? That could be worth .5 to 1 mpg at highway speeds.