[Ptero]

Just a comment. You can't makle a princess out of a pig. You don't use the truck enough to justify putting money into it for eco mods. Get an economy car and occasionally use the truck for what it was made for. If you must use the truck, air up the tires to sidewall max and drive 50 mph in high gear. You most likely have a lock-up tranny in a 2001 so accelerate very slowly into high and stay there. (Use the same technique for a non-lockup.) Don't let it kick down pulling grades - back off the throttle and keep it in lock-up high as long as you can. Some models require you to reach a base rpm for lock-up. You'll have to figure out where this is.

That is the number one thing you can do. It's free and easy.

If this is the engine you have
5.4-liter, 3-valve Triton™ V-8 Engine
it reaches a torque rating of 90% at 2250 rpm
which means your best economy is probably around 1400 - 1600 rpm and you will never have a good reason to exceed 2250 rpm pulling heavy loads aside from time constraints and traffic.

If you don't have a tachometer, borrow a hand unit and write down your rpms at speed. Then, on your regular run, determine your optimum economy driving speed. A vacuum gauge hooked up to the intake manifold will really help. You'll figure out what it is telling you about your economy technique real fast.

When pulling a trailer load, make sure the trailer tires are at max sidewall pressure. Accelerate as slowly as traffic will allow. I mean, take a mile to get up to speed if you can. Smoothly and steadily. Really. This is the big method to minimizing fuel use in hauling.

On the road, use constant throttle technique. Find your foot position for say 50 mph and hold it steady in that position (or less) for the entire trip. Let your vehicle determine the speed on grades. Use this technique for both loaded and unloaded, truck, car or motorcycle, and you will see a dramatic difference in fuel use.