Brand New Deep Cycle Batteries Keep Dying Overnight

Much better and easier to use a clampon ammeter.

Disconnection and reconnecting a circuit may change/reduce the draw due to a reset condition and hence it's easy to be mislead about where the draw is occurring.

RVs have parasitic draws that cannot be turned off. My inverter cannot be turned off - sleep mode is not off. My salesman switch turns some items off and the Monaco battery disconnect switch turns off more items but still have a battery drain.