Most likely the WFCO Converter/charger is always producing 13.6VDC to your battery for a long time frame. This always started my batteries to boil out fluids. Of course once that starts the battery life is shortened big time.
The fix is to have smart mode converter/charger that works. It needs to drop back to 13.2VDC when just sitting there and this will not boil out fluids like the 13.6VDC potential will do...
I would replace the WFCO converter/charger with a PD or IOTA unit. The WFCO items doesn't get good reviews from the RV'ers maintaining smart mode charging...
I changed my new WFCO WF-8945 Converter out for a PD9260C Converter when I lost a battery due to boiling out fluids in 2009. My remaining three batteries are just now needing to be replaced for low performance... Not bad for 6-7 years of heavy use I reckon...
Roy Ken