What prairefire is talking about is called a galvanic reaction. I forget the chemestry behind it but it operates sort of like a battery in that when the two dissimilar metals touch eachother it creates an electrochemical reaction that causes the electrons to migrate from one metal to the other (I think) which causes oxidation (rusting).
Ah, found this
The idea is that on the galvanic scale, metals that are further apart on the scale will have faster corrosion rates. Metals that are closer together will have slower rates of corrosion.
Sometimes the galvanic reaction can be used to improve corrosion resistance such as with galvanized steel. The layer of zinc plating corrodes instead of the underlying steel (I think).
With all this being said, i can't figure out what different metals your gas tank is reacting too. It's a steel gas tank mounted on a steel chassis with steel nuts and bolts.
It sounds like it's a simple case of the contact rubbing off the paint coating and exposing bare steel to the elements.