Ally, have you tried disconnecting the batteries and checking the voltage of each battery seperately with a multi-meter?
A good, fully charged and settled battery should read 12.6v when disconnected from the vehicle (open circuit test).
Your 12.1-12.3v from your batteries makes me wonder if something is leeching power from them? Or perhaps a bad connection? Or low electrolyte level? Or maybe one of them could have a defective cell. (even a new battery)
I know nothing of our camper power needs right now as we haven't used it much yet BUT I was a bit of a "voltage & amps" heure on our sail boat...
A rule of thumb I always ran by was -
If the battery is reading
12.6v its 100% charged. If that battery drops to
11.5v it is 0% charged or effectively dead. (12.1v would be 50% charged). It's not good for a battery to draw power from it when it drops below 11.5v..
Second rule of thumb -
A 12v 25watt bulb draws 1.5 amps. (Useful to know when you have an amp meter.
)
Our boat leisure batteries would read anything up to 14v with the engine running and once the engine was shut off, the voltage would gradually drop down to 12.6v. Switch on a light (25w main cabin light) then the displayed voltage would drop to 12.5v. Switch off the cabin light and the displayed voltage would quickly rise back to 12.6v.
The only time our leisure batteries would drop to 12.1 -12.3v would be if we had run them down a bit having lights on for several hours, or LOTS of lights switched on at the same time...
I'd recommend fitting a digital amp meter for more useful info.
