Had a 1980 3/4 ton Chevy 4×4. It had a 4:10 rear end. It only got 8mpg just sitting in the driveway! LOL!
I simply went down to Tractor

Supply and bought a 50 gallon (?) transfer tank.
I mounted it in the bed. Doubled the range of the truck. Rigged up a valve to swap tanks inside the cab.
1) fuel bill doubled $$$$
2) driving around with a firebomb in the bed of my truck
3) my wife and kids drove that truck
All DIY!
Bolted tank down securely to bed.
Added drain valve and a moisture trap.
Added fuel filter.
It worked great.
Ripped down a 2×4 to a 3/4×3/4 stick and marked it off in one on increments. Like a dipstick! Each inch contained a little over a gallon.
We ran off the factory (30 gal) tank then swapped to the big tank.
Add a lock to the cap. Youngest po'd a classmate that "sugared" the tank. The moisture trap and drain made flushing the tank easy.
Right now, I carry two 30 gallon drums and a transfer hand pump.
Gives me +/- 90 gallons.
Adjusting for expansion, I carry about 85 gallons. At 15 (or more) mpg, I can get about a 1200 mile range.
Prefer the two diesel drums to the 50 gallon gasoline tank!