Strange brown staining on Cap Decals

[socalhd]

2014 Fuzion FZ 342 Fifth Wheel Toyhauler. Took delivery of it 18 months.
My rig has developed and ugly brown/rust looking stain on the upper Front cap decals. This crap wont come off and I have no idea where it came from. I do a good job of keeping it cleaned and waxed.
Any thoughts?

[gearhead]

I get some mildew on mine.
As I recall the weather on Rosecrans Avenue you may be getting some effects of your occasional morning fog and settling dust. There are a couple of experienced detailers on this forum and they may respond.