In addition to selecting bank 1 or 2 or all a battery selector switch has an off position. If you see smoke and fire coming from your engine room, immediately turn the switch to off- it is outside of the engine compartment isn't it in case it is an electrical fire. Your 50 amp house breaker won't accomplish that purpose

And is 50 amps the right size. That breaker is to protect the wire from the batteries to the main panel and as such it needs at least #6 wire for 50 amps.

David