Why??? I've been driving propane-powered pickups and farm tractors since they were first popular back in the 1950s. Most of those pickups were dual-fueled - they ran on either gasoline or propane by flipping a valve. I haven't had a propane-powered pickup in a lot of years, but I still have a 50-year old Massey-Ferguson Model 50 that has never used anything but butane/propane fuel. Propane-fueled engines burn cleaner, and with comparable maintenance, will last at least twice as long as the same engine running gasoline. Witness my of MF-50 that has never had an engine overhaul. But they burn about 20% more fuel and have less power than the gasoline version. And right now propane with the road tax paid costs about the same as gasoline with the road tax paid.
Back in the 1950s, we ran untaxed "tractor" propane in our pickups. It cost 7 cents per gallon while gasoline cost over 25 cents per gallon. So it was worth the risk to run the untaxed propane to save a buck. But the internal revenue folks at both federal and state levels didn't appreciate us using untaxed fuel, so they clamped down in the 1960s. I don't know about propane - nobody uses it in farm tractors now - but the "red" untaxed diesel in your pickup tank can get you a huge fine.
I love my old MF-50. It's retired now - like me - and doesn't work more than about 20 hours per year. But I would much rather have a diesel engine than a propane-powered engine in a truck, or in a working farm tractor.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.