Here is what I think:
On my RV (1990 Winnebago Warrior, Ford E350 chassis), the Charging Solenoid, which gangs the house and chassis batteries together, is activated 1 of 2 ways:
FIRST WAY:
Momentary Position of rocker switch:
This uses power from the HOUSE BATTERIES to energize the solenoid. This is a "jump start position", and is to be used when the engine battery is dead. Thus logically you would want to use the house batteries, and not the engine battery, to energize the solenoid.
SECOND WAY:
DUAL position of rocker switch:
This uses the engine battery (or alternator if engine is running) to energize the Charging Solenoid. But, it is hooked up to the RUN position of the key.
When you START the engine, typically all other accessories are OFF. I can see this when I start my engine as all of my aftermarket gauges turn on when I switch to RUN, but turn OFF when I keep turning to START, and turn back on again when I let go of the key and it goes back to RUN.
My guess is that the Charging Solenoid is only energized when:
* Rocker Switch is in DUAL mode.
* Key is in RUN position.
