Your excessive voltage drop indicates high resistance and your converter/charger also sees that high resistance so it "thinks" the batteries are nearly full, so it does not go into boost, but goes to its 13.6 stage. 13.2 indicates it thinks the batts are full and it has gone into "storage" mode during which it should do the occasional 14.6 shot to destratify the electrolyte.
Your batts may indeed be as full as they can get, but that may only be to half what it was. Or you have a huge wiring problem somewhere that has the high resistance like a loose connection. Something is wrong somewhere.