I installed a Fan-tastic vent on the top of my jasper blue 140 hitop. I don't remember seeing any surface rust.
Is it just brown dirt or is the paint really chipped off to expose the metal? What color is your van?
I have read about this before, but it's normally just road dirt.
The only place that shows any form of rust, and I haven't looked at everything closely, is the rear door hinges. But that's to be expected because the paint just scrapes off in that area.