I have never used Zinc anodes on a truck but 1 small one in front and one in the rear, I think would take care of everything in between. 2 things to remember, in my experience, they take a long time to start working. 2- don't paint over them, and if you bolt them on make sure the touch bare metal somewhere. They are not a cure-all against corrosion, but a definite helper in the long run.
