Does the voltmeter draw that much power? If not, is there something undesirable by it being left on?
And if the supply voltage to meter is what is displayed on the meter*, most likely the voltmeter isn't going to work on only 0.7 volts.
If the power supplied to power the voltmeter is different than the voltage measured, then you could put a couple of diodes in series with the voltmeter power supply, thereby dropping the 0.7 volts to nothing. When the car is powered up the voltage supplied to power the voltmeter would be 11.4 to 13.6.
You could also do this for the condition indicated by the asterisk*, but your measured voltage would be lower than reality.
I think what you are seeing at those fuses is standby power. The car really doesn't have a power switch or a truly OFF state. The start/stop button is pretty much that, not an on/off switch.
Think of a modern ICE vehicle. if there was no standby power, how would the door locks, the alarm, and telematics work?
I have a hunch that the 0.7 volts radio circuit would be fine for your voltmeter. Have you measured the internal resistance of the voltmeter?