Yes, the 12v items will still be powered by the onboard converter while on shore power, in most cases. Some RV's will need the batteries in the circuit to power the 12v system correctly, they buffer the output of the converter's power supply.
Your stand-alone charger will work with batteries disconnected by the switch if you have the charger's leads connected to the (+) and (-) terminals of the batteries. Hooking the leads into any other wires while the ground is interrupted, will not complete the circuit so no charging will take place.
12 volt side of life