spiffytexan:
This is a project that somebody has to do. Pickup trucks are not wildly popular without reason - their crummy MPG and all. Here on the sweltering steppes of central Indiana, Chevy half-ton pickups are as ubiquitous as traffic barrels. (Indiana has two seasons: Basketball and road construction).
So far we have determined that you have a 4x2 truck with either a 4.3 V-6 or a small V-8, an automatic, and at least 4.10:1 gears.
Is it a short bed or 96" bed?
If you continue to drive fast, the biggest hardware gains you can make are in aerodynamics. Can you live with a tonneau cover? I had a hard A.R.E. flat tonneau and I saw a 1.5 MPG improvement right away. Better yet would be a "fastback" angled top, but nobody makes them (a guy on here is close). I fabbed a 17 degree sloped (too steep I know) bed fairing and it gained me another 1.5 MPG over the flat tonneau, and a full 3 MPG over an open bed.
Half-ton Chevvies are easy to lower. There are all sorts of "slam" kits for you. I lowered my Ford 4" and gained 1 MPG.
Since my pix are too big check out this link.
Dave Whitmer's Amazing MPG Machine
Now maybe my Ford is ugly. Sorry, I ain't Chip Foose. but I am getting sumertime FE in the 27 MPG range. If I can do it with a 1-ton, what can you do with a half-ton.
At this point your gas-pig is so egregious that you have lots of low-hanging fruit. As everyone else has pointed out, adjusting the nut behind the wheel is powerful and cheap. Ride that horse as far as you can before you go spending big bucks.