What happened to the propane rigs so popular a few years ago?
Propane/butane powered pickups were popular from the 1950s through the 1980s because many farmers/ranchers had propane-powered farm tractors, and they could easily cheat and use untaxed "tractor gas" without much chance of getting caught as a tax evader. The city boys never did use propane, because they didn't have a convenient source of untaxed propane. And very few farmers bothered to convert their cars, because the propane tank took up the whole trunk. And besides, it was an expensive conversion.
But if you have to pay full retail price including road tax for propane, then it's not cost effective. Propane has about 25 percent less BTU per gallon than regular gasoline, so your "gas" mileage is a lot less with propane. And in most places, propane now costs about the same as gasoline.
But the main thing that killed propane as a pickup fuel was when diesel tractors replaced gasoline or propane tractors, and diesel-powered pickups replaced propane-fueled pickups. The farmers no longer had huge 1,000-gallon propane tanks from which they could conveniently fill up the pickup with untaxed propane.
Back in the 1950s and '60s, our pickups were dual fuel = butane and gasoline. Gasoline was about 30 cents per gallon and tractor gas butane was about 7 cents per gallon. So we used a lot of butane (propane) in our pickup. Because of the worse gas mileage, we rarely bought taxed propane when on the road, because the cost-per-mile was less on regular gas than on taxed propane. The pickup ran just fine on eigher fuel, but got a lot better MPG on gasoline.
Farmers and ranchers don't run the red tractor diesel in their pickups, because the revenooers look for pickups with red in the fuel tank, and the fine is very, very high when you get caught.
I still have a 500-gallon butane/propane tank I use to heat the house, and I fill up my old propane-powered Massey-Ferguson Model 50 tractor from that same tank. But I don't have a pickup that will burn propane now - just diesel.