I took the voltage for my voltmeter from the battery and from the line to the CO and propane monitors, using a switch to select which source I wanted.
I selected the monitor line because the current drain is small and constant, and easily accessible, so that gives me a good indication of the converter output.
The battery is wired directly to a 12v socket that is only used for the computer, usually when driving, or for a Battery Minder when being stored over the winter. Usually this socket is unused so it is a good indication of battery voltage.