ALL metal corrodes, just some less than others. Stainless by definition is a low corrosion metal, but it CAN corrode. 316L is a great stainless metal that is used in applications such as the food industry for this reason, but over time with water, acids and salts, it can corrode.
Depending on the grade of stainless, you get more/less protection from corrosion. So you might either have a lower grade of stainless, or you have chromed carbon steel. I'm sure the guys here have a better idea of what stainless grade was used in these trucks.