[ray_mcavoy]

Trucks with dual tanks have a "dual tank wiring harness" that plugs into the "base equipment" fuel sending unit wire.

So to convert back to a single tank, all you have to do is unplug the base equipment sending unit wire from the dual tank harness and connect it directly to the sending unit (it should already have the necessary plug). The connector you have to unplug (and move to the sending unit) should be located along the frame somewhere near the tank switching solenoid valve.

All of the dual tank specific wiring harness can either be left in place (unused) or removed as desired.

Finally, if the fuel gauge still doesn't work after plugging the "base equipment" sending unit wire directly onto the sending unit, you'll have to do some additional troubleshooting to see if there is a bad connection somewhere or a faulty sending unit or gauge.