John, I'm guessing from your description that you have one main onboard tank and the"City fill" thing on the outside of the van. We never used this and could not get our heads around the concept.
So just a quick thought that may help a little but not a total fix. On the effected tap have a delve inside the base unit and where the wires from the tap go the choci-block terminal, disconnect them at this point so the tap is isolated. You should then still have the other tap fully functioning. This is not ideal, but might get you out of trouble.

Colin