Just had a similar problem with my aux battery. Luckily I got a
warning from the battery monitor which indicated that there was very
low voltage after a short period of use without engine running or
solar charging. My LED panel under the stove also showed a low state
of charge. I put the smart charger on it and it came back up, so I
thought maybe the battery was not getting enough  juice from the
alternator. With the engine running, the voltage was a bit low, so I
cranked up the adjustable regulator to 14.2 at the battery. That
helped, but the problem persisted. I yanked the battery out, took off
the caps (level OK), hooked up the charger again and it went to
de-sulfation mode. All cells began to boil except one, so I tested all
with hydrometer. That's when I found the dead cell. It was only 2.5
years old and never run down, but my battery guru says it happens once
in a while. Lucky me:-(

Moral of the story: take good care of your batteries and if there's a
problem, don't wait to find out what it is. Could cost you a new
alternator AND battery if you put it off.

My 2 cents...

Keith O