My roof is aluminum and for some odd reason, very clean, but when I do wash it I'll use the long handle brush while standing on my scaffold or a tall ladder.
The only reason for the original prep is to get rid of the layers of crud. My MH looked like it had a crackle finish. To get rid of the black mold in the seams and crevices I used a tooth brush and mineral spirits followed by scrubbing the whole rig with BKF and a good wash down with Dawn and a rinse. The final wash with Dawn is very important to get rid of any powder residue from the BKF.
After applying the RMP I did notice, months later, black streaks running down the side after a rain. I simply sprayed it down with plain water and rubbed with the brush. Gone.
When you get back from a trip, from what I read here, just hose down the cab-over front to get rid of the bugs. No scrubbing.
Finally, if you don't wait for the MH to look dull again all you would have to do is wash it down with Dawn and re-apply the RMP.