FYI just killing power to the Magnum remote isn't enough to stop the standby loss since the inverter's parasitic current is more than the remote's.
I am using a Blue Sea ML-RBS Remote Battery Switch Solenoid with Manual Control model #7700 to disconnect the batteries in my rig. It is rated at 500 Amps continuous duty (700A intermittent).
I have it wired on the positive lead just after the batteries. It's ahead of a Magnum 3000W inverter and also the main lead for the coach 12V circuits.
This is a slick setup. The remote switch is inside mounted adjacent to my inverter panel so when storing after a trip I can completely disconnect power to the Inverter and all 12V circuits by simply pressing a button. There is no need to open any compartments or the battery box.
The ML-RBS also has a mechanical interlock lever so power can be locked out safely if you are doing service work.
It uses a mechanical latching design so it doesn't consume any power itself other than when you are pressing the on or off button.
If you don't need the remote capability then Blue Sea has manual disconnect switches rated up to 500 Amps which would be suitable for up to a 3000W inverter.