To be certain, you should:
* Consult your owner's manual: This is the best resource for the precise location of your truck's components.
* Look for a diagram: A repair manual or online diagrams for your specific year and model F-250 might show its exact placement.
* Visually inspect the area: Look along the passenger-side frame rail near the fuel tank. The valve may have a cable connected to it leading to the inside of the cab.
Because it's under the truck, you'll likely need to get under the vehicle to find it. Be sure to use jack stands and follow all safety precautions when working underneath your vehicle.