I suspect the capacity of those batteries, given their age and charging regime, is quite low, but if you only drain them a tiny amount, you wont really notice how little capacity they have. With such a low rate of charge, the batteries are probably seriously sulphated, with minimal capacity to handle large or long duration smaller loads.
I regard my large battery banks as like an old steam engine with a big fly wheel, not a lot of revs but plenty of torque. We have just had 8 days of rain with very little sun and all banks maintained their voltage. For the past 12 years we ran a farm stay business that catered for 10 to 20 guest at a time with 2 fridges 1 freezer and occasionally a small cool room.
As Smurf1976 said
If the batteries have survived 28 years then the charging can't be that bad, can it?
But to get back to my original question of is there something else that I can do to make what I believe to be a very safe system even safer. I am a little confused by the Acronyms used and not sure that the safety devices suggested can be used with out an earth.
Peter
