RE: The out-gassing possibility of the standard automotive battery, coupled
with use a propane heater like the Propex is enough to give me pause. Is
this gas flammable?

The Propex takes all combustion air from outside the vehicles and all
exhaust goes outside the vehicle.  No interior air gets near the ignition
source.  Interior air is only pulled through a heat exchager, total
seperation from the ignition source.