If its purpose is to isolate the house batteries from the starting batteries during cranking and when the engine is OFF, it will only be ON when the engine is running and you are trying to charge the house batteries from the engine. I am certain that your engine alternator can not generate anything near 300A !
In my RV, there is a momentary switch option to gang the house batteries to the engine batteries during cranking, so that the house batteries provide an emergency jump start if the engine battery is dead.
I think engine starters can pull 200-500A if I'm googling correctly.
