The 12VDC Distribution panel gets its 12VDC from both the Battery and the On-Board Converter/charger unit if it is plugged into shore power or generator.
I always have my batteries on-line when on shore power. They make a great buffer for the converter circuits and will smooth thing out. Also gives you more capacity for the higher current demands like the jacks or the slide-outs...
Of course in our use of TRAILERS the battery is required by DOT to have while towing the trailers to activate the ELECTRIC BRAKEs on the trailer in the event the trailer becomes disconnected from the tow truck. This would not be a requirement for the Truck Camper.
My camping world revolves around camping off the power grid in OFF-ROAD mode so my batteries are the most important items I have...
Roy Ken