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Hi my name is Robert...
I see no other experts have responded. IF you had a battery isolator you would have one cable coming from the alternator to the isolator. Normally the input will go to the center of the isolator and you would have an output on each side, one going to the chassis battery and one going to the house battery. The voltage on all three posts should be the same or very close.... I would try and trace the cable to that isolator. Some will not charge the house battery via the alternator and some only charge the house batteries once a certain voltage is sensed on the chassis battery..... Robert