Is the battery warm to the touch ?
I have two batteries in the 5th wheel. I've had this happen twice and each time it was due to a bad battery. Because of the bad battery, my charger/converter kept charging and boiled over the two batteries.
If your battery is more than 3 years old I would suspect it has gone bad. Have your battery tested and replaced if bad. This should solve your problem.
Keep in mind it is possible the charging feature of the converter has gone bad and isn't switching to a trickle charge when the battery is near full charge.