I would wash then try to spot wax it. Just use a wax recommended for the siding of your unit. Most will say not to use on a decal but I have lightly waxed the front decal on the cap and it has not hurt it. You might also try a "magic eraser" but be careful on the decals. I have a feeling it might rub them off if you push too hard. You might also try 1 of the spray on "black streak" remover products.
It is probably a leaf stain rather than rust. Leafs transfer oil from the host tree. It is organic try soap and water first. If that is not enough try a water and ammonia solution. You can also search organic cleaners.
Al Laura (Lily & Breeze Portie Girls)
2011 F250 6.7 PS Tuxedo Black 4x4
11 305SKP- TPMS- RideRite air bags
Reese 16K Slider Wet Bolts