i think is comical you are going through such an elaborate cluster to complete such a simple task, but i guess what else would i expect from the dumpbus saga. bitching this much about such a simple task, eeek
seriously, what about doing it electrically automatically? does the oem fuel gage function properly with some type of resistance sweep? could you have like 2 resistors, a cap, and other penny cost majic to drive fet to pull a fuel pump relay on and off automatically when a threshold is met on the main tank. could have a manual over ride switch also.
i guess that would not allow you to know your big tank is empty and or run a transfer pump dry.
thought: same thing with solenoid valve electric or air valve that would not run dry