If you are sure the converter isn't overcharging the battery then get the battery checked, maybe a shorted cell
I had a converter on a motorhome that would overcharge but only intermittently so it was hard to figure out. I think- it was a while ago- that I disconnected the converter charging wire, or pulled the fuse and charged the battery with a trickle charger that I plugged in to an outlet in the MH.
I hope this helps.
Frank