In those systems where the tank can be filled by the trailer water, there is
actually a valve that bypasses the pump for just that purpose. Water cannot
flow through the pump backwards, so the fill water is introduced between the
pump and the tank, flowing backwards for a short distance into the tank.
The tank fill nozzle cap should be left off for this operation to prevent
overpressurization of the tank.
GQ '67 Safari
4082 in CA