2 - 6 volt batteries or 2 - 12 volt batteries or...

If you have 2-6v now then they are in series not parallel. Given you camp with hookups then I would just replace with a single 12V marine/deep cycle AGM battery and call it good. You wont have to monitor water levels like you do with the 6V.

I would figure out your charging situation however as you will likely just toast the new battery. TO leave hooked up to shore power you need a charge controller that can step down the voltage/current when the battery is fully charged.