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[SmokeyWren]

There are several options to increasing your fuel capacity. But be careful of "cheap" fuel tanks. Be certain any fuel tank you add to your pickup is DOT approved. The good stuff is made by TransferFlow, and is not cheap. You can "make do" with lesser brands if you don't mind doing some pumping of the fuel yourself instead of having a fully automagic system tied to your fuel gauge, or with a fuel gauge that doesn't tell you how much fuel you have left.

It sounds like your best bet would be to replace your 30-gallon tank with a bigger one. The 46-gallon replacement tank from that link posted by Y2KPSD above is probably what you need.

Another option is an in-bed auxiliary tank. Several choices there, from 30 to over 100 gallons, including just a tank, or a combo tank/toolbox, or an "L" shaped fuel tank in the front of the bed with a toolbox sitting on the lower part of the tank.

When shopping for fuel tanks, understand the difference between a replacement tank, an auxiliary tank, and a refuel tank. A refuel tank is more common, because it costs a lot less. Refuel tanks are most often used to haul fuel to farm tractors or construction equipment, but they also work fine to haul extra fuel for your pickup. It is simply an extra fuel tank, not tied into your fuel system. If you want to use the fuel in the refuel tank, you have to stop and transfer the fuel, using a pump of some sort.

For a replacement or auxiliary tank, I'd go with TransferFlow. But for a refuel tank, almost any brand will do. The important part of a refuel tank is the pump that would transfer the fuel from the refuel tank to the truck's fuel tank. The really-good 12-volt electric pumps are not cheap, so you wind up paying almost as much total (including the pump) as for a TransferFlow auxiliary tank.

There are no factory optional fuel tanks on a SuperDuty pickup. Your shorty has a 30-gallon fuel tank, and it will actually hold about 32 or 33 gallons when you fill it up to where liquid diesel is visible at the cap. If you get only 20 gallons in your stock tank when the "empty" light is on, then you aren't filling it up. After the automatic pump shuts off, you can then dribble more fuel in through the foam. It takes me 5 to 10 minutes of dribbling to add the extra 5 gallons or so that will go in mine after the automatic shutoff. But some truckstop diesel pumps won't allow you to dribble it in - it's either a bunch or nothing. In that case I usually just delay filling completely up until I find a different pump that will allow me to slowly dribble in the fuel slow enough that the foam melts as fast as the flow makes new foam.