The reason a 6V relay is used is so it always has full voltage. If you use a 12V relay and the voltage drops below 12V it may not stay energized.
I am really having trouble coming to grips with this statement.
Typically a 12 volt relay will energize at 10 volts or below and stay energized at 9 volts or less. Since the engine would not crank at a low voltage, then there should always be enough voltage to pull in a control relay if there was enough voltage initially to start the bus.
A six volt relay subject to a continuous 12 volts or higher, depending on the charge rate, would have a seriously reduced life expectancy.
Electric dash panel gages on older cars used a voltage reducer so all the gages would always have steady voltage to give accurate readings should the input voltage fluctuate. Most reduced 12V to 5V.
6 volt ignition systems reduced the voltage to 5 volts thru a dropping network to the instruments. I do not know what the 12 volt systems do.
The coil on 12V gasoline engines is actually a 6V.
The engine is only started with 12 volts on a 6 volt coil to produce a hotter spark. When the ignition key is in the start position. Once the key is returned to the run position the voltage to the coil is routed thru a dropping resistor to drop the coil input voltage to 6 volts.
At least this is the way I understand it.