Tim,
There’s no secret to shutting it down.
Make sure no shore power going in and generator is off and breaker in bay is off.
Make sure no loads are drawing power in the coach.
Your 2 red switches on the curbside engine bay are marked 12v and 24v, turn them off. that shuts off battery power.
You should also have 2 red switches on the driverside in the inverter bay that allows you to bypass the inverters and it has an off position.
In your Marathon manual you can see a picture of the 2 red switches in the bay.
To power back up have no loads and turn both red switches on.
Later
bv