The smog technician could have been referring to the filler neck... fill stem is not an official term for any specific emission component. He/she may have been referring to the filler neck, which is where the gas nozzle is inserted when fueling.
The smog test requires an inspection of the filler neck to ensure it is free of obstruction, of the correct size (as to not allow leading gasoline from being filled into the tank), and that along with the gas cap, it provides for a good seal. If either one of these conditions where not met, your Ford E150 could have failed the smog inspection.
We can not say without diagnosing your Ford E150 (or at least seeing it) what may have caused it to fail the smog inspection for a problem with the filler neck. A filler neck failure is not common, and is usually only when the vehicle has been tampered with.
We recommend returning to the smog station where your Ford E350 failed and inquiring further on the failures specifics. You may need to visit a test and repair smog station after that, however at least you can receive a bit more insight into what exactly caused this smog failure and which component the smog technician was referring too when he/she mentioned filler stem.
