12/24v Solar Controller --> 6v Batteries

Background: Purchased a used coach, came with a solar panel/controller setup that's connected to Two 6-volt house batteries. I got no paperwork for any of it but scouring the interwebs I at least found out what kind of controller is in place (a Chinese brand w/little info in the online manual). Literature describes it as a 12/24v unit.

I'm FAR from an electrical expert but it seems like the batteries are getting heavily charged during the day (boiling or near boiling the fluid... can audibly hear them peculating louder than what I'd expect. Seems like something's not right with the system but again, I'm not an electrical guy & never had any solar home or RV equipment before now.

Question: Can using a 12/24v solar controller lashed up to a solar panel send too much voltage to 6v batteries effectively killing them prematurely?

Pretty sure I need to replace these 2 house batteries but I don't know if I should go back to the OEM setup of Group-24 12v types or stay with 2 6v types.

Thanks much for any assistance you guys can offer!