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I would be wiring it to the AUX battery via a suitable fuse as suggested , leave your start battery for that, just starting.
Don't need a flat battery out in the Simmo
Check condition of both batteries if any are older than 3years I'd be looking at swapping it for a new one.
You don't need any dramas out the middle of no where
Perhaps wire a relay in to accessory so you don't have to remember to turn it off.