If this is a lead acid battery that has the removable caps you can look to see if one cell bubbles faster than the other, (eye protection please) also owning a specific gravity test bulb is cheap and you can test cell by cell for THE CORRECT SPECIFIC GRAVITY OF EACH CELL in lead acid batteries. Taking the battery in to a battery vendor with a sophisticated battery test set up
will be free...Bearing in mind they are in the business of selling them.
If the average age of the employee staff is under 20 and using one of those drop load testers then find another vendor.
I prefer using the specific gravity of solution method. Do the test after the charge.
My bet is you lost a cell, one will bubble faster than the other.
You will see a two point drop in cell specific gravity between one of the cells.
Happy Camping, Safe Trails.