Verify from the manufacture, they should be able to tell you. If it has an oily residue it leads me to believe it is not galvanized. If it is galvanized read up on coating them. Galvanized metals need to be aged prior to painting and make sure you use a coating that is designed to paint it. What you don't want is to use a paint that would allow moisture to penetrate the paint and rot the tank out. It would be better if you left them alone. If it is just a steel tank with a rust preventative I would find a good 2K epoxy, get a marine grade. Tanks will outlast the truck. My $.02