Bad battery - maybe short which would make battery boil over, maybe neglect from not providing periodic water, maybe bad converter which went bonkers and put out too many volts (low probability).
Your going to need a new battery - should also test your converter to make sure it's putting out correct voltage.
Do you know your which model converter you have? Old fashion converters tend to put out constant voltage which tend to deplete battery water requiring constant replenishment - newer "smart" converters don't deplete battery water very much and are considered wise investment by many RVers as they don't cost much, easy to install, and save batteries.
Hope this helps.
