@David Analog
That makes sense. I actually have 3 group 27's. 2 of the 27's are just for the stereo, with the first being for the boat.
What seems to always happen is that I end up running the stereo bank about dead, or at least to the point where the system starts to cut in and out.
After that, I usually do the no-no of combining the banks, even though one battery is fully charged, and the other 2 are dying... Not sure exactly how bad that is, but I'm sure it's not good...
Then when the stereo starts cut out again, I take that as a sign that I need to go home.
I probably need to add another battery I guess. Or maybe another bank???
For three total group 27s, two of which are discharged deeply, a 30 amp charger is not too much, although borderline. A 20 amp charger is a bit lite and isn't enough for expansion. A 30 amp will allow you to add one more group 27. A 30 amp charger is likely to be limited to a 3-bank model even though you only need two and will only use two of the banks.
First, get a digital volt meter to bury in the battery locker (so you can read it in the daytime). Never discharge your batteries below 12.0 volts, which is half charge. The occasional 11.8 volts would be the minimum for a consumer grade deep cycle battery. You will get an inordinate number of shallow cycles. But as you discharge more deeply the number of cycles will plummet. It's not a linear ratio.
I would not combine my deeply discharged stereo batteries with my starting battery.....ever.....AFTER a long stay at rest. Beforehand is fine.
You don't want to start the boat and operate the boat electronics with a lower voltage as it is extra hard on the starter & boat electronics.
The heat generated by suddenly combining the two different banks at wildly different charge levels is bad for the starting battery. It's not as much of an issue on the stereo bank side because you have multiple batteries to dissipate that violent current transfer.
You do not want to combine the load of several deeply depleted batteries, a large stereo at play, plus the boat operations, and place that considerable draw on the boat's stators. They'll run as hot as an exhaust manifold and that's unhealthy.
Again, allowing the boat's charging system to service the stereo batteries and running both banks in parallel is okay throughout the early day and before a duration at rest. After that time, it is better to keep the stereo bank in isolation, based on what voltage you read at the end of your stay.
It sounds like you have worked your batteries pretty hard thus far. Before adding another battery, I would have the status of you old batteries checked. The new battery will degradate quickly if introduced into a bank that is way past its prime. Optimumly, batteries within the same bank should be identical is chemistry, size and age (even production date). You can get away with a late introduction of a new battery if the existing bank is young enough.
