This is interesting - I've often wondered how to assess the remaining capacity of our four lead acid batteries. In the past I've used a Sterling battery tester to check condition, but this is only really a good test of cranking ability, and doesn't tell me what capacity is 'left' in the batteries.
We've got four 110 Ah batteries, and cruise daily for around four hours. The engine-driven alternator is rated at 70 amps (max). After four hours the output to the batteries reduces to around 3 amps, telling me that they're near full charge. We have 200 W of solar through an MPPT controller too (for sunny days !). Overnight we use around 50 Ah (according to our Victron BMV unit). When the batteries were new, the voltage would fall to say 12.4 v overnight, now after some 100 discharge cycles (100 days), the voltage by the morning falls to around 11.8v, which I know is not helping.
How do I test to ascertain the remaining capacity of the batteries ?