Dead battery?

Checked the fluid levels in my batteries yesterday. The 2 newer ones on the house side were nice and full. The one independent battery which is my port side starting battery (Interstate RV/Marine deep cycle) appears to be older. Popped the covers and could see the plates were exposed. The battery took almost 500mL of water. It was looooow. Ran the generator for a while to charge them up as I had noticed them discharging faster over the previous few nights on the hook.

Then this morning I get woken up by the CO detectors going off and I have no idea why. I unplug them and go back to a restless sleep. When I get out of bed a little later I discover the voltmeter on my panel shows 8 or so (290 DA, not sure if it shows system voltage or not). It's so low my phone isn't even charging in the 12V receptacle.

Is it a coincidence that I had this issue after I added the water? Is that battery toast? I was concerned it may have boiled over on the 3 hour ride home but that didn't happen.

Thanks

Once plates are exposed battery is not good. I know happened to me.

Also, when a battery drops below 11 volts it pretty much is junk at that point. It will never retain a full charge after that.

How old are they? Marine batteries have an average life span of about two years thats why they only have a one year warranty. And when you added water was it distilled? And why did you disconnect you co detectors and go back to bed there is a reason they went off not a good move.

That's a bummer. I hate battery problems.

Yank them all an have them tested.