The rest of the vehicle's electrics remain on 12V (lights, radio, aircon compressor, etc.) that is acquired by by tapping a supply off one of the batteries only. I guess you could also tap 12V off the other battery for your fridge if you like living dangerously?

BTW: The Delco Remy 21 SI that I installed is also available in a 24V version. The vacuum bit it does not have
