Way back in the 1950s we converted our farm pickups to butane (another name for propane). Actual to dual fuel gasoline and butane. The reason was because untaxed farm butane cost seven cents per gallon and gas cost about $0.29 cents per gallon. The pickup got about 25 percent less MPG on butane than on gas, but it was still cost effective.
We also had butane-powered farm tractors. For the same reason. They burned a lot more butane than gas, but the total fuel cost was enough less that the savings would pay for the conversion is a short time
I still own a late-sixties Massy-Ferguson model 50 highboy that is butane only. It hasn't had a tune up for about 15 years, but it still starts right up if the battery is charged. It's pretty much retired now, used only for smoothing our long gravel driveway with a box blade after a rain. And it doesn't rain much here, so it doesn't get cranked very often.
But the economics of using butane/propane are not the same now. We use propane to heat our house and for the water heaters and kitchen range, so we have a 500-gallon tank out back. Propane costs about the same as gasoline now, when delivered in bulk at 300 to 400 gallons at a time. So there would be no savings in fuel costs. And if you have to fill the tanks at RV propane fillup places, propane costs more than gasoline. There would be a tiny bit of savings because you'd have to tune up the engine less often - you might even go 15 years between tuneups like I have on the Massey-Ferguson. And the motor oil stays much cleaner, so you don't have to change the oil as often. But given all that, if I swap my big generator for one with a different fuel than gasoline, it will be to an Onan diesel. Regardless of maintenance and oil change intervals.