[77newell]

The recommendation from Newell has always been to replace them together. As batteries age they change their internal resistance. If you have batteries with significant age differences the differences in internal resistances will effect how they individually charge with the higher resistance batteries charging slower. The other reason for changing at the same time is that generally if one has died due to age then the others probably won't be far behind.

I have not personally always followed that advice using the logic that I was always plugged in or running the generator. It did require a work-around once or twice when a battery died and discharged the other one in the pair (my setup had a pair for the chassis and just one for the house). Once this lead to both chassis batteries freezing and remarkably recovering when charged (miracle do happen but don't count on it). Those batteries each had one cell that was lower than the rest for the next 4-5 years before one completely died and I replaced them both.

My thoughts at this time is to replace just the bad one if the the batteries are fairly young. If the batteries are older I plan to replace them together. The older the battery the more likely it is to fail. If the failure is due to an internal short then all the connected batteries will tend to discharge. As long as the charging capabilities can keep up with the discharge rate AND the discharge rate is small enough not to generate significant heat you may decide to live with it. Each of us has our own tolerance and judgement of the risk reward. If your case were my case I would be buying all new batteries since they each seem to have a bad cell. The real question here is when I would do that and since it seems that so far it really isn't causing you any issues in using the coach. If you were me I would have a very firm plan as to how I would handle the situation when they do die at the most inconvenient time.

If you are always plugged in you will never need the capabilities provided by deep cycle batteries. In fact, if you really are always plugged in or on the generator I would be thinking about reducing the number of house batteries. With my previous coach I got by with one 8D house battery. It was never a problem given how long it was the sole source of power. Just because your coach was built with a particular number of batteries doesn't mean this is the right number for the way you use it. You could need fewer or more, it sounds like you need fewer.