I think they died and that's why they went down to 11volts. The new ones, which are used more than the old ones, are on 12.5 or more in the morning and that's with the temp higher so making the fridge work harder overnight (it's on the lowest setting before you ask).
But I am still paranoid about batteries. I don't know if they were knackered when I got them or if they were OK and I killed them. Or if I treated them well but there is something wrong with my charger, generator, alternator or solar panel and one of them killed them.
They were 7 years old but the boatyard that put in the new ones said they should have lasted 10 years or more.