Instead, if it's a dual-tank model, the selection is done via a selector switch (often a rotary knob) usually located on the dashboard. This switch activates the fuel pump for the selected tank. There's no separate valve; the switch controls electrical power to the appropriate pump.
If it's a single-tank truck, there's no such switch or valve.
If you're troubleshooting fuel delivery, focus on the selector switch (if applicable), the fuel pump, and fuel lines.