I just bought a '92 F-150 with dual tanks. The truck was undercoated from the factory, so the undercarriage is in excellent shape.
The problem I am experiencing is when I switch from the front to the back tank, the gauge switches to indicate a full tank like it should, but within a few miles the gauge reads empty and the front tank reads full. Also, the truck begins to act as if it is out of gas.
The problem has to be, or at least related to, the fuel tank selector switch. This guy does all the work. When you're in "front tank mode" the selector switch sends 12 volts to power the electric fuel pump in the front tank. At the same time it connects the fuel gauge sending unit in the front tank to the fuel gauge in the instrument cluster. When you select the rear tank it does the opposite, and hooks up the pump and sending unit in the rear tank.
It sounds like there may be bad contacts within the fuel tank selector switch causing you grief. If it's acting up on a regular basis get it to a technician. It should be a quick confirmation with the appropriate wiring diagram and a digital volt/ohm meter.
Here's a little trivia on a somewhat relative recall pertaining to '90-'93 F-series trucks equipped with dual tanks. There was an issue with an internal check valve within the fuel pump not working properly. This would allow fuel pressure to "cross-flow" to the tank containing the bad check valve, which could eventually cause fuel to actually overflow out the fuel filler cap -- not fun.