Welcome Morgan, I'm in Prineville and work in Redmond. And I have a 76 F250 4x4 Hi-boy truck. My fuel valve is fixed so when it is pointing to the back of the cab: it is on the main cab tank, pointing at the drivers door; it is on the left tank and pointing at the passengers side; right tank. If your valve is under the cab on the drivers side; follow the lines and see which tank it is drawing from.
First, I would make sure there is sufficient gas in each tank to fire it up. If there is, try starting it with the tank selector in each position. If it is still acting like it is out of gas, it probably isn't the selector valve. Even if the selector solenoid is bad, the valve to one tank or the other should be open allowing fuel out of the one tank.
Check the fuel pump before you get too carried away.
Look at the floor between the edge of the seat and the drivers door and see if you have a valve there. Our farm truck has the "Aux-Main" Toggle switch on the dash, but it only controls which tank the gauge is reading. The tank flow control valve is a manual gate type valve on the floor at the above location.
My 78 was all electric but it was a rocker switch labeled "Front / Rear" as is my 96.
If you are 100% electrical, you are going to need a multi-meter and you are going to have to make sure the connections are corrosion free and the fuses are all good.
Oh yes, Welcome aboard and be sure to visit the BS thread.