Hi...
Sounds like a bit of a nightmare..
Try fully charging the battery bank and then disconnect at the the battery bank..Not at the breakers..
The voltage will initially be quite high,13.5-14.5v but should settle to about 12,6v or so after a few hours...
If no further drop is observed then something is amiss in the yachts systems, possibly between the bank and the breakers..
If the voltage drop still occurs, then there's a problem in the bank or one of the batteries........Good luck....Bill.