If one or both batteries became ill, the charger would not stop charging because a full charge could never be achieved. Then boil-overs and warm batteries occur.
Like yourself, I had a pair of 12V Walmart batteries. I got two good years from them, one fair year, and one poor year. Prior to the Walmart's, I had the batteries that came with the rig when new. The story with both pairs was identical.
After repeat experiences with pairs of 12V batteries, for the last replacements I got a pair of 6V AGM batteries from Sam's Club at nearly twice the price. Year #3 has been just as good as year #1. You'll have to ask me one & two years from now how they are doing.
One thing I noticed about the 6V AGMs, they hold more energy so I am not charging as frequently. There is an improved "Quality Of Life" that comes with the change-up.
One additional item I added which helps, was installing a volt meter in plain sight as shown. When electrical operations are at rest and the meter reads 12.1V, it's time to charge the batteries.
