[C&P RV]

It sounds like you still have your converter (12V) still hooked up to one 6V battery which is not good. If that's the case, I'd turn off the circuit breaker for the converter or disconnect that battery until you can connect the two 6V batteries in series again.

In my experience, if you have two batteries (12V in parallel or 6V in series) and one gets weak, you will have a better long term experience replacing both at the same time. The old battery will "drag down" the new one pretty quickly.