On our R27-OB, there is nothing that the boat came with to tell us the voltage of the thruster battery. The chart plotter will give us engine and house voltage. The thruster battery is a mystery.
We installed a Victron 712 Battery monitor. It monitors everything going in and out of the house bank, which helps us a lot now with understanding power management. The Victron has a lead that goes to the house voltage. so I can get a better voltage reading off the house bank without using the chart plotter.
For the thruster battery, the Victron allows a second lead to monitor voltage of the starter battery. I connected that to our thruster battery instead. I'll know if I have low voltage to the starter battery when I hit the ignition and nothing happens. I don't need meter to tell me that. But our thruster battery was installed in a very hard to get at spot, not easily accessible to even put a volt meter across the terminals. Using the Victron battery monitor to read voltage on the thruster battery made sense.