Anything you can just plug will not be very accurate. It will be some version of a volt meter. Voltage is not that useful tellin gyou the condition of your batteries. A fully charged battery under a heavy load will have a low voltage. A poorly charged battery under a light load will also have a low voltage. A fully charged battery will have a high voltage, so will a bad battery hooked to a strong charger.
To really know how much battery capacity you have left you need a meter that includes a shunt wired inline with your large main negative cable. This measures amps in and amps out over time. You tell it how many amp hours your battery bank is and it calculates how full it is.