two batteries on the hitch is fairly common. It is either 2x12V batteries in parallel or 2x6V batteries in series. If the camper is the same as when purchased new, then it is likely 2x12V batteries in parallel. IF there is not a battery disconnect switch or dial, then you'll want to physically remove the wire connection from the positive battery post of both batteries. IF there is a switch or dial, then use that to disconnect.
If the propane sensor has its own backup battery, then I'm not sure how one disables that. But mine will be completely shut off once disconnected from a 12V power supply. My father's however will alarm even when the main camper battery is removed because of how his solar panel is wired to the system. The solar panel can provide enough power to the detector to run it, and also trick it into thinking there is low battery and therefore alarming when solar input is low but not off.
2012 Aspen Trail 2710BH | 280 watts of solar on the roof | 2x6V GC batteries | 100% LED lighting | 1500 & 300 watt PSW inverters | so far strictly boondocking