If a 12 volt battery gets below 12 volts, its time to charge. If it doesn't make it to at least 13.5, its time to change. Either way, its a good idea to check the calibration of the display device against something of known accuracy occasionally.
In a multi battery set-up, it is a good idea to make sure that the battery being reconnected is the same voltage as its mates, a small difference can give a very big circulating current as they even out. If they connect through a schottky diode unit, the highest voltage one carries the load until its voltage drops down to equal the next, then they bot carry the load.