For a $500 propane powered bus you will spend almost as much or more converting it back to gas as the price of the bus. Consider you will have to buy and install a gas tank, carburator, gas lines, ignition module, and re-set the timing.
You might consider leaving it propane powered. Sure fueling is problematic, or just a regular PITA, but if you fuel at every opportunity, you shouldn't have much problem.
It might be worth noting that while in the clearlake area propane may not be a lot cheaper than gas, in other places it is.
Here are a few links on the subject:
The number of refueling stations by state
The location of all refueling stations (mapquest like functions)
Heating oil and propane prices:
Gas and deisel prices: