My starting battery dies when the main house battery switch is on, whether on shore power or unplugged. With the switch off the start battery stays charged. All batteries charge with the engine running. All batteries are about three years old. I suspect the isolator, but cannot locate it, not sure if it even has one. On top of the start battery are two small boxes ( Hi Amp.,Buss 150 amp,. 181150F) I suspect they are circuit breakers, but have found them called isolators, circuit breakers or diodes ??? If they are circuit breakers and faulty,(open) I would have no power. ..... Where would an isolator be located? Any suggestions would be welcome.