Thinking on this more, I think my biggest issues is the connection of the alternators. Connecting dual alternators is a common issue with big powerboats with two engines and you should be able to find examples there.
You have the
alternator output connecting to the argo, which splits the charge
current to all three banks. The voltage sense is either at the
alternator, the argo, or the start
battery. The problem lies in the different
battery banks. There isn't a smart
regulator so the alternators are pushing full voltage. More like a car. Even if there was a smart alternator the voltage sense cable is not on the correct battery or location. You either don't see the correct voltage at that location or your only seeing the voltage off the starter battery.
As other posters are stating the starter banks only need a trickle charge. So they are full quickly. Your house bank is not so lucky. 6volt cells will boil if charged to much, losing all their electrolyte, and as it's an acid can cause damage if it
leaks somewhere. If the car style system is just pushing a set voltage it will damage your house bank over time. Worse if you don't continually check your electrolyte and say the level drops an inch, you lose capacity, even if you catch it and top of later. You destroy your
batteries over time.
If there isn't a smart
regulator the voltage sense is the alternator terminal usually. Resistance in the
cables means it sees a lower voltage than what's actually at the
batteries, specifically the house bank since it's farther. That means the excitation voltage is throttled up to push more juice to the battery. this can boil or explode a battery.
If the voltage sense for an alternator is on the argo. Your closer but it's a similar issue. The starter bank voltage will overcome the house bank. If I am understanding things correctly this will overcharge your starter batteries and undercharge your house bank
If the voltage is on the start bank than it's only seeing the starter voltage. Seeing that quickly tops off your house bank really gets under charged
I deployed for 6 months at a time. I have a smart regulator. The
charger would be left on when I left. 24v system with 6volt batteries. The electrolyte evaporates and drops anyway. Come back I topped off. Couple years of this and my battery capacity definitely dropped over the years. You don't get the ah capacity from all that exposed lead plate surface back. Life of the bank dropped got 7 years roughly and have to replace now. Should have lasted 10-12years. Your 6v cells are more expensive than mine. Trust me it hurts the pocket book replacing those things early. If you ever switched battery chemistry your system definitely would not
work. Just fix it correctly now, simplify and protect your batteries.