I wouldn't have a caravan now without the on board tank, my system works different to WCamper, the on board tank has its own pump, the level sensor does NOT stop the pump, only gives a indication that the tank water level is low.
Advantages
1/ water is generally warmer so hot water tank goes further.
2/ if outboard tank runs out, just flick a switch to bring the onboard tank on line, no night time trips to the water tap.
That's the system on my 2004 bailey.
Not sure how the on board pump system works, one pump and change over valves?