As TitanMike said, a lot of little parasitic draws on those house batteries (to add to the list: radios with memories, appliances with memory settings, CO2 detector, etc.). Most if not all of these devices SHOULD be on the correct side of the disconnect switch so they should not get power when the switch is OFF, but . . . .
Also, you didn't mention your battery capacity, but there's 720 hours in a 30 day "month". That means just a 0.1 Amp draw (not that much) drains a 100Ah battery of it's usable capacity (50Ah) within three weeks
If your batteries are smaller (or perhaps not fully charged), it may happen even faster or with an even smaller draw.