Use a mop to get the worst off, then use bleach (allowed for rubber roof) to get the black spots off. Spray 100% laundry bleach in an area, let it turn yellow and the black spots turn grey, mop that, hose it down, move to a new area to spray. Repeat if the black spots don't go away completely. Takes a while but it works great.
Hose off the sides of the rig after too in case the bleach bothers the decals. Not seen a problem there myself, but it is a standard warning, so included here
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1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.