Well, a whole list of questions so -
1. I measured across each battery when it had no load - because I disconnected cables on one post - so I could see if there was an imbalance, and there was. I am a liveaboard so there is a continuous cycle of use and recharge, either from the 512w solar or from the engine alternator (100a).
2. I said 'apparent' good state of charge because I could only use the voltage indicated on an MT50 monitor unit. With no charging going on from the alternator or the solar I wold expect 12.6 - 12.7v to indicate a good state of charge and I used to see it quite often but in the last 3 months it became rarer.
3. Since I bought the boat two years ago I have tried to keep the non-charging voltage above 12.1v at all times, but over the last 3 months this was very difficult, in the morning it would be down to 11.8 or even 11.6v. In the winter of 2017-2018 the boat was left moored at a broker until I picked it up in mid-March and I suspect there was little or no charging of the batteries in that time which won't have helped their longevity, the old ones were at least 4 years old, perhaps older.
I had an electrician look over the system last year and the charger was OK then. He had me run the engine at 1100rpm for 3 hours and at the end the MT50 was showing 14.6v. I tried the same thing recently and only got to 14.3v. Because of the rewiring I got a professional in again to swap the batteries, test the starter battery, and check the system was working right for the AGM batteries and he gave it the thumbs up.
So far I don't use shore power at all but I'm aware I don't have built-in battery charging if the boat is plugged in to a shore unit so I am considering swapping out the 17 year old Sterling 3000w inverter for a combi inverter/charger with lower output, say 2000va, and add a proper monitoring unit like one of the Victron BMV range to give me a better idea of what is going on.