Excuse my bad spelling. Dual battery. Can't change it from taptalk

I have a redarc 100amp isolator and have been throw 2 deep cycle batteries. In 4 years. The last battery has only been in there for 2-3 months and won't charge above 7.5 volts. I have only run the engel fridge from it for one weekender in that period.

I have checked the voltage when charging and it is always above 13volts.
Given this is a 3l the battery sits quite lose to the turbo with no shielding. Is this likely to be killing them.

I have not tried charging this new battery with a 240volt charger but the old one didn't recover.

Any ideas??