It is extremely unlikely that your zinc problem is related in any way to the 12 volt system inside the boat. You are tilting at windmills, fixing the wrong problem.
Your first line of defense is a Galvanic Isolator
in the ground lead of your shore power
connection. It is only a $100 expense and can pay for itself in one year of zinc savings.
Once it is installed, measure the AC voltage across it. The paint
bubbles you describe are a symptom of AC leakage that forms bubbles of sodium hypochlorite under the paint. If the AC voltage is greater than about 0.5 volts you need to troubleshoot problems in your AC wiring
and add a Galvanic Capacitor across the Galvanic Isolator
to bypass the AC current
Rather than trying to isolate the battery
system, there is less chance of DC electrolysis
from the battery causing problems if ALL the negative connections are solidly bonded to the hull. That way they have to be at the same voltage as the hull and minimize introducing extra problems.
I lived aboard a steel
boat for nearly 15 years, send me an email
if you have any questions.