If your van uses Whale water components, then the tank is filled by pumping from your aquaroll, via the sidewall connection, until water exits the overflow.
To fill the onboard from the pitch supply, you will need a Whale Watermaster hose/connection kit. The connector block simply fits to the same sidewall connection point, but has clips to hold it place, to prevent mains pressure forcing it off. This is because you will find a 2 way tap between the sidewall inlet, and the onboard tank fill point. The second position stops the tank filling, and instead pressurises the van water system. However, the Watermaster block contains a pressure reducer, which has the effect of reducing the flow-often below that of the onboard pump. For this reason, we use ours just to fill the onboard tank