I don't think I would say "normal", but I doubt if its a problem
Probably just spots of some sort of dirt or mold
a lot of folks use spic and span, or laundry soap
maybe try a weak bleach solution if nothing else touches it... just rinse really really well and rinse the run-off too on the coach sides....
use a good scrub brush on a handle....
The key is finding the right cleaner for the whatever the dirt is....
your dirt is probably different than my dirt, so what works well for me may not cut your so well....
__________________
|