Your battery is screwed, it is shorting out internally and producing gobs of hydrogen gas.
As someone else posted, put it somewhere (very carefully) well ventilated, keep away and just let it die a miserable lonely death.
Don't try and recharge it, just get rid of it.
Try for warranty if it still had some left but they will usually only offer part of replacement if anything.
Your voltage readings are perfectly normal so thats good.
That solenoid linking the Batteries is OK, old school but still OK.
I agree with TD in that if you do want to update to a modern battery parallel management system you are half way there because you can use the existing cables.
You can go Redarc SBI12 for about $100 to $150 depending on what C/B's you choose to protect the battery feeds if you don't have any already.