I agree with the first statement.
The gauge has it's own power connections inside the dash.
The gauge switch is inside the valve not on the dash like the 73-80 aux tank systems.
When you flip the dash switch the polarity of terminals D & E is reversed and the 12v DC motor inside the valve reverses till the shuttle trips both the overtravel switch stopping the vave motor and selecting the proper sender.
B is the common on the sender switch inside the valve. The wire from B goes to the gauge.
C & A on the valve are connected to the tank senders. If I drew it right A is the RH tank and C is the LH tank.
The tank senders are 0-90Ω variable resistors (specifically rheostats). One end of the sender coil is connected to the gauge and the wiper is connected to the tank. The tank is grounded to the frame by a short little chunk of wire.
Your problem is somewhere between the RH frame and terminal A on the valve.
The wire colors to the senders and the gauge in this drawing likely aren't correct but the functions are.
