For hard core survivalists or just people who live in hurricane areas, the advantage of propane is it never goes bad. Gasoline can only be stored for a year, diesel can have problems with algae growth, propane will still be good when the container holding it rusts away. Great to just put a big propane tank next to your whole-house emergency generator.
For RVers, if you are trying to stay out in the boondocks, I suspect you'll run out of propane long before you run out of gasoline, IF you stop and fill your gas tank before leaving the highway.
That said, I guess propane is cheaper than gas, although it also has less energy per gallon, so I don't know how those equations play out. Oh, and there are some people who claim that an engine burning propane has less wear than an engine burning gasoline. Don't know if that's true or not, myself.