Adding Inverter for dry camping

I have an inverter in my fifthwheel and use it when Dry Camping. I have two batteries in parallel. My question is when plugged in to shore pwr the converter is charging both batteries, the converter is a three stage one and steps down as batteries are charged, now if one battery has a bad cell and cannot fully charge will the converter see this and continue to put out maximum charge and not step down, thus possibly boiling the good battery.

I was trying to find a battery isolater howerver the ones I find on the internet are for use with an alternator and not sure if you can hook the converter to that as a charging source