It is wired so you can use the house batteries to boost the chassis batteries or the other way around. The boost is temporary while you hold the switch the switch energizes the solenoids that act as electronic jumper cables. One of the solenoids acts as a battery isolator.
Another feature is to allow the alternator to charge the house batteries when the engine is running and there is a time delay.
It sounds like your chassis battery is dead and once you start by using the boost the alternator and house batteries are providing power.
You could have a totally dead chassis battery or one of the solenoids is bad.