there is or was a bus in the for sale section that is propane powered.
Propane is heavier then air, so it would seem that any minor leaks would not go up into the bus. This is assuming the tanks are outside of living space and are for running the bus engine. However if propane is used in the bus lets say for the stove then of course a leak in the bus could be a problem, and would be wether or not a woodstove is in there too.
A woodstove is great, but some care must be exercised or it is a fire hazard all by itself.