My opinion ... Neither the battery nor the propane should be stored in an unventilated area. Either one alone has the potential to cause issues if an ignition source is present. If you are getting sparks from any source other than a 12v switch, relay, or brushed motor, then you are having problems with your electrical system which needs to be corrected. That being said I can't see an issue with having both in a well ventilated compartment (both upwards for the hydrogen from the battery and downwards for the propane) with NO switches, relays, motors, etc.
Also, considering the construction of propane tanks, and automotive batteries, I can't see a situation when the presence of one would dramatically affect the results if you have an incident with the other. Either way, it would be a BAD day.
Corwin