My first surprising is a bad ground. Then one battery with a weaker cell. How is your bilge pump or pumps wired? Sometimes you can have a circumvention of a combiner on the back end that will drain the batteries to an extent. (I trust the water levels or "usage" is non-remarkable? ). Do you have a bilge bump counter? You might have something odd with the solar panels or their controller. Someone once had a discharge from one when that seemed incongruous. You aren't leaving an inverter on in standby mode?

As Steve points out, 82% is not 12.1 volts. That seems like a red flag to me of a connection issue.

I would disconnect the batteries from ANYTHING and each other after you believe they are charged. Then, after letting them sit overnight, I would check them for voltage at the batteries and with a hydrometer. Then I would then connect them up. See what the voltage reports on your panel and Link. You should see the same results