Hi, I am not a battery expert so I am attaching a word document that I “copied” from somewhere in my travels (Not sure how accurate it is and sorry to the original author/source for not giving them named credit). The document may provide some good general information about battery charging and determining the true state of charge of your batteries. It is important to make sure you are fully charged and that the batteries will actually take a full charge as a starting point (not damaged). (The document provides info on terminal voltage and specific gravity measurements; also make sure, if you are using wet cell batteries, that the water/acid levels are slightly above the top of the battery plates. It is possible that chargers fail to charge properly or they can also overcharge and boil batteries dry.
If batteries are in fact fully charged and are not damaged and they continue to lose charge for no apparent reason, you likely have loads still attached even when you think you are disconnecting the batteries. It is possible for the coach and chassis battery disconnect latching relays in the BCC to fail “always on”. Of course you should see signs of this with devices in the coach still on (like gas detectors etc), if the batteries have enough charge and are in fact capable of powering anything at that point.
If your coach has a BCC similar to the one I have, the interconnect relay will typically operate to connect the coach and chassis batteries together if either the coach or chassis batteries are adequately charged and the appropriate coach or chassis latching relay is operated to “on” position. I believe that using the coach battery to start the vehicle, because the chassis battery charge is too low, would require that the coach disconnect latching relay is “on” and that the coach batteries are adequately charged to operate both the interconnect relay in the BCC and to also start the vehicle.
Last comment in case it is helpful; for my refrigerator, even when it is operating on propane only, there must be 12volts provided by the coach battery/converter to allow the fridge thermostat and its basic electronics to work. Verify with your refrigerator manual as this is another source of battery discharge, although a small one compared to running on electric mode only with gas off.
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