Voltmeters are handy but remember they are not the best way to determine the health of an FLA battery. FLA = flooded lead acid; any lead acid battery with removable caps. The reading on a voltmeter can be higher than it should be or lower than it should be. If the battery has been recently charged the voltage reading can be too high. If the battery has had a large discharge recently the reading can be too low. The voltage reading on a battery can be false if the battery is being charged or being discharged. As was pointed out if there are batteries in parallel a meter will not be of any use to find a problem. The more cells in series the less useful as well. I see a voltmeter as useful but not the end all answer. FWIW, I think wiring in selector switches to be a waste of time because a hydrometer is the only accurate method of determining cell health. Either a hydrometer or a refractometer.
We have 2 six volt batteries in series in our trailer. The old RV had 4 six volt batteries in series / parallel configuration. Our off grid cabin has twelve six volt batteries in a 24 volt series /parallel configuration. All have a single voltmeter used as a general guide. Each is wired in with a switch that allows the meter to be switched off. I have the switch mainly so I can turn the display off and not have the bright light from the led readout shining light when we want it dark inside. The current loss to the meter is very likely less than the self discharge rate of an FLA battery. All cells in those batteries have their sp gr read every even numbered month. The cells in the cabin batteries also have the readings recorded. I used to use a paper notebook, I now speak the readings into my tablet and transcribe to my Excel later. I have a six year history of cell readings to keep track of any changes in cell health over time.
The best hydrometer and the best price on it that I have seen.
@ NAWS