all here have given you some great tips. I may not be adding any additional information but reinforcing what has been said already.
I know your young and when I was young, I was always in a hurry. being in a hurry with a big vehicle will waste gas. take you time, leave earlier for the next destination, relax and enjoy the typical speed limit and maybe +5. but at 75, you will burn lots of gas compared to 65.
accelerate moderately and let off gas so that you cruise with the torque converter locked up (not slipping which is wasting energy). you can feel the torque converter lock up and unlock. if you are taking your time, you can try to drive so that it doesn't unlock. such as when starting to climb shallow hills, you might have to back off the throttle just a bit and go up slower to keep it locked up. remember that it is sensing torque and if you have to exceed the torque limits set in the computer, it will unlock. try to keep it locked up as much as possible. of course, if the hill is steep or very long, this is not likely possible.
pump up tires above normal pressure. remember, your'e taking your time and wont need every last bit of traction in the tires. lowering rolling resistance with proper tire selection and keeping it a bit over-inflated helps.
some here got extreme with their trucks and added a cover like thing that streamlined the entire back end. works but you may not like the look (I don't) but that is a personal choice. improving aerodynamics a bit does help, even in big trucks.
most of your gains will come from driving technique. after you get that down good, then the aero mods help achieve more gains.
again, take your time. life is good, enjoy it! want to go fast, get a go kart and go to the track to get the racing out of your blood. safer.
Thanks for all the tips guys! I've been busy with school the past week, so this is the first chance I've had to be back on here. When I get the TC fixed, I'll start logging in my FE and we'll see what mods net me the biggest gain. Again, thank ya'll!