Tuga,

Voltage is only one indicator of a battery problem. Obviously a battery reading as low as 2 volts has a problem. While four years is not a very long time in battery life it is much closer to the end of life than the beginning.

Not knowing your coach I don't know if the batteries are used as 12 volt in parallel or 24 volts in series. It makes a difference because if used as 24 volts apart from the battery problem there might be an equalizer problem.

Yest terminals need to be kept clean, the top of the batteries also so voltage doesn't leak to ground through a trail of corrosion.

To test your batteries they do need to be disconnected regardless of whether they are wired in series or parallel. I would not only test for voltage, but I would use a battery tester to determine the battery condition. A battery tester will tell you if the battery needs a charge or is junk. Before testing the one with low voltage I would charge it separately and see if it even takes a charge and holds one.

You say they are wired in series, so just so there is no doubt, in series it means one negative post on one battery is connected to a positive post on the other.