The manual switch between the batteries and inverter is also useful for ensuring ALL phantom loads stop when you flip the switch. My rig still pulls some power even when the main switch is turned off, so I added a 1000W manual switch that will stop it all.
This may be less important if you have an automatic way to maintain your batteries while in storage, like solar.
Which brings me to a question. The Magnums are also 3-stage chargers. Does that mean it's ok to leave the rig plugged in long-term during storage, or is it still better to disconnect the batteries entirely and maintain them manually. (Assuming no solar yet, and AGM batteries so no worry about water.)