I think the 12v battery only gets a couple of charging amps since it is not intended to do very much.
Maybe this might be of some related interest, (More)
Power to the People: Run Your House on a Prius. However, this story is about drawing directly from the HV battery, and not the 12v battery. Though I have my doubts that with only 3kwatts, one can run everything in the house except for the central air.