Those trucks are not made of…
Those trucks are not made of a true stainless steel. The are made of 302 stainless steel which has a higher content of carbon steel in it. Thus it tends to rust as if it had no paint on it. They chose this type of stainless so the body panels were easier to form in the stamping process. If they chose 304 stainless steel, it has less carbon in it but making it harder to stamp the body panels. A true stainless steel would be 316. which doesn't rust whether it is painted or not. This is all something the manufacturer won't tell you.