question

Sorry, you're showing us the wrong bits. The 712's shunt goes in a common negative feed to all the batteries. Similarly the positive for voltage goes to a single common positive across the batteries.

So to install you may have a tricky rewire to do. Or multiple 712s.

Looks as if you have a common positive on the top pair. But the windlass battery looks like a standalone.

You would need three independent BMVs, one for each battery bank. All connected ahead of any switching. Some of the BMVs have a second voltage input but that won't give you state of charge for the alternate battery.

The shunts for each BMV would connect in line with negative terminal of each battery bank BEFORE any chargers or loads. That is the only thing that connects to the shunt negative is the battery (or batteries if two are in parallel).

If I were you I would take the opportunity to make a cleaner installation.

Set one battery as a pure starter battery and make the others as pure house batteries. Have the solar panels charge the house batteries only as the starter battery will be charged while running the engine, and that quite fast if the engine is not too difficult to start.

Doing so will make it much easier to be in control of how much power you have left in your house bank.

In my boat I have set it up in that way using 6 house batteries and one for start with solar panels charging the house batteries only. I have a solution with a relay that connects the start battery with the house batteries when engine is running and by that also charges the house batteries. I also have a charger that charges both the house batteries as well as the start battery when connected to shore power.