Battery disconnect switches are a MUST IMHO. Below I will include a link to the ones I use. As common in the industry, connect to the main negative from the battery, one for each battery.
Now, lets trouble shoot your issue. Connect a known good digital meter to the battery connections connected to the battery. You should observe a current draw, dropping your battery voltage. Now disconnect, one at a time, circuits until the volt drop stops. Usually, when you find it the volts may even go up some to recover from the draw.
Another test to consider requires disconnecting the positive at the battery. Connect the meter to the positive cable and the other lead to the ground. With no volts showing, set meter to ohms. It should be zero. Any other reading here will confirm a positive to ground condition, usually suspect with a grounded hot wire some where, or some component failure.

Amazon.com: Kohree Battery Disconnect Switch, 12V-48V Battery Isolator Master Disconnect Power Cut Off Kill Switch Waterproof with On Off Switch, and 4 Mounting Bolts for RV, Atv, Car Marine, Boat, 300/1000 Amps: Automotive