my last stepvan ran on propane.
due to the fact that there's no crappy additives in propane like there is in our gasoline (or whatever that swill is that they sell us)...you could take the intake manifold off, and the top of the engine inside was absolutely CLEAN and SPOTLESS!!!
I got my propane at our local U-Haul truck place. I've also bought it at gas stations, truckstops, and campgrounds too. They nearly never mentioned road tax to me. (except at the gas station.) I did have a fuel oil place refuse to sell to me, but that only happened once. (but I just think the guy was just being a powerhungry jerk.)
I too paid about 2/3 the price of gas, but I did use a bit more then I would had I been running on gasoline. (if I had gotten 13mpg's with gas, then I'd get about 10mpg's on propane.) Also, as propane burns cooler than gas, it doesn't generate as much power, so if you were loaded and were climbing a big hill...you wouldn't have the same power to work with.
One other thing, is that if you run outta propane out on the road...it's towtruck time. You can't quite fill back up along the road using a jerrycan like you can with gasoline.
I must say, though, that I did learn to love it!