One of two chassis batteries not charging while driving

[ds28]

I bought a 2012 Winnebago Vista 30T , my first purchase of a motorhome, and I remember the seller telling me how he replaced one chassis battery with 2. At the time I didn't quite understand the purpose and thought he added a 2nd 12volt battery. However, I believe he replaced 1 12-volt battery with 2 6-V batteries. They are in series. While plugged in , or generator running, they both read around 6.7 volts.

While driving though, after an hour or so, if I stop, the engine barely restarts, or I have to hit the button to get extra from the coach batteries. After my last 1 hour drive, I checked both batteries. The one reads 6.3V, the other 5.4V. Plug them in and they both charge up again.

Did the previous owner cause this issue by going to 2 6V batteries? Or do I need a better alternator?

Thanks in advance.

[vsheetz]

Are the pair of 6v chassis batteries wet type? If so, full of water? A hydrometer to check specific gravity will give you an idea of each batteries condition.

Dunno why he replaced a 12v starting battery with a pair of 6v deep cycle. I would put it back to a single 12v maintenance free.