My suggestion is to add another A-B-BOTH-OFF battery switch to the
existing house battery + terminal.

Yes, the trick being finding an effective location to mount said switch.
The existing battery panel in the port gunwale doesn't have room for another
switch on it. So I'm rummaging around below to find a suitable place to add
this A-B-Both unit.

The bilge pumps are on the starting motors, and are always live (through a
breaker). The starting batteries have a relay that allows bridging across
all three banks should one of the starting batteries be too low to crank
it's engine. Even if both starting batteries were dead the switch will
still pull from the ships system. Thus far I've never had to use it.

I hear your comments re the Rule connectors, pieces of crap.

So I'm still wondering about an isolator and whether this fourth bank needs
one. I have absolutely no desire to get into flipping the A/B switch to
deal with charging. Since there's no good place in the cockpit it'll likely
mean this switch would have to be placed below, in the engine compartment.
All the more reason to avoid changing it.

Tangentally, I found marine 1/0 wire from SkycraftSurplus.com for just over
$2/foot. Quite a bargain.

-Bill Kearney