Are you sure that it is 'overflowing' and not simply that the tank drain has been left open?? The drain fitting may be well hidden but will be very near the tank itself and letting water flow out below the tank and, hopefully, on to the ground.. The drain may be as simple as a capped (or uncapped) piece of pipe sticking out below the vehicle.
On some RV's there may be a valve that allows the tank to be filled from the city water connection but that is not a 'bypass'. It either fills the tank OR allows the connected water line to service all the plumbing without using either the pump or tank. Some vehicles have a 'gravity fill' inlet for the tank as well, while some may have only a gravity fill inlet for the tank. Both will have a tank vent somewhere nearby the tank on the outside.
