Is there any danger in storing the propane tank inside a larger aluminum box that will also house the battery and fuse block?
I am of the mind that certain things are not worth the risk. Several years ago I was on the design team for a piece of military equipment. We had extremely tight overall size constraints as it needed to fit within the confines of a V-22 Osprey cabin. We arrived at a decision to mount, what is roughly akin to a shore power charger in relative proximity to the AGM batteries. The space of the batteries and charger were not shared however were within the same EMI shielding zone.
According to the cut sheets and the OEM's engineer of the charger, there is no way a spark can be created. According to the battery data and interview with the OEM, there is zero off gassing from the selected batteries. Well guess what...during testing, under a high discharge, rapid re-charge condition the EMI cover blew off with enough force to imprint a chain link fence. Right, can't happen. Instant project delay and rapid redesign and a sigh of relief that nobody was standing near that side of the equipment.
If you were to take advice from an anonymous person on an internet forum, separate and isolate batteries from vaporous fuel sources. While batteries and fuse blocks themselves don't spontaneously give off sparks, they are vulnerable to being shorted by accident or loose object. Propane cylinders are not in business to leak but valves and fuel lines can fail. Having propane in an enclosed space with a potential sparkler isn't best practice. Vent the bottle and isolate the electrics.