I think you have two problems, maybe not related: the starting battery
boiling dry and a wiring problem. First the wiring problem.

The prevalent wiring scheme for most boats straight from the factory is a
starting battery wired to the "1" terminal of the 1/2/Both/Off switch and a
house bank of one or more batteries wired to the "2" terminal of the switch.
All house loads, starting and charging are wired to the common terminal of
this switch. So, if the switch is in the Both position everthing is all
hooked together in parallel.

You wrote that the reefer, a house load, is independently wired to the
starting and house banks- "draws from both banks simultaneously, and cannot
be isolated to one
bank or the other ". This is very weird, because it negates the switch and
can allow the starting battery to be pulled down by the reefer load at
anchor- not good. Or any other house load, like lights for example can do
the same because the reefer connection always keeps everything in parallel.

So, if it really is wired that way, then the simple solution is to wire the
reefer load only to the common terminal.

I doubt that this will solve your starting battery boiling dry, but it will
at least give you the ability to do something about it. When set up like
above, good batteries and chargers don't behave this way, particularly if
they are all the same type of flooded cells, which they must be for you to
see the electrolyte level. I suspect you have a bad starting battery.

David

David