I had the same thing happen. When you lose shore power the parasitic loads drain the batteries down to nothing with the battery switch on. I changed my storage strategy. Now I have the battery switch set to OFF and have a charger/maintainer on the batteries. I have mine permanently mounted in the battery compartment hooked up to the batteries. If power is lost the charger just stops and switches off. No parasitic loads.
I have a
Noco Genius but any good charger/maintainer should do it. You just need to make sure it completely disconnects and puts no load on the batteries when it loses AC power. No point in adding parasitic load.