RE: REMOTE FUEL FILL VALVE
A fuel valve as you describe is for a two line tank with the line from the fuel valve to the tank going to the clunk line. The second tank line is the vent line to the top of the air chamber in the tank. It simply gets routed to a convenient place where when the tank is being filled, either it is allowed to pee on the ground or into a temporary connected overflow bottle to indicate when the tank is full when filling.
The vent line is a critical part of the system as unless air is allowed in/out of the tank, the tank can't be filled/drained.