As a general rule I occasionally monitor my voltages directly at the batteries.

I have found that the on-board gauges are slightly incorrect, possibly because of the distance between the gauges and the batteries.

Our batteries for both the house and chassis have the ability to make a good trip into a nightmare and have done so on my previous coach. I now know having learned the hard way that if the batteries are checked while the bus is running, while being charged, after resting with no loads overnight, and after having a load applied for a while I can usually know one or more are going bad, or that they are OK.

The key is to check them regularly and after a while you will know what the normal reading is and when a reading is out of the norm it gives you an opportunity to investigate and correct a problem prior to a trip.