My guess is the fuel filler is behind the driver side door, below the door handle. The filler tube passes through the cab behind the seat, and exits through the floor to the tank below. Dodge put a lot of fuel tanks under the driver side floor, inside the frame. Unfortunately, it is also below the front of the box. When the wood box floor rotted away, the rain water sat on top of the tank and made a big rust hole in the top of the tank. Lots of tanks were pulled out to get fixed or replaced that never happened. If the tank got pulled, likely the filler tube is gone as well, but the 3" diameter hole in the cab corner and the hole through the cab floor should still be visible. Gene