I don't really bother "trying" to keep the outer tops of the bunk ends clean other than wiping them off with a towel on a extended pole as we break camp. We camp in state parks where we get tree sap here and there because I always get a shady site.
I wish I would have taken a picture. We have a hybrid where we camp 95% of the time in shady state park campsites. As you know if it's 80 + degrees and sunny out, those bunk ends can heat up fast. At my work I came across some heavy-duty bubble wrap where one side was white and the other side was super shiny silver colored. It was like a mirror on the silver side. They came in on a shipment of peanuts, wrapped around each pallet. So each piece was 10' wide by 25' long. They were discarding it, so I took some home, cut a piece out to fit over the bunk top area, enough to "clip" down.
One of the campgrounds we go had a tree problem where one of our favorite sites is now full sun. When I used the silver bubble cover, it greatly reduced the temperatures inside the bunk ends allowing the AC to cool the inside much easier.
In closing, I try to minimize our set up time, so spending 10 minutes each time trying to cover up the bunk ends just to keep them clean, I don't see as valued time. We have it down to about 20 - 25 minutes to completely set up.
Our 2005 hybrid
we had 380 nights in, and when I sold it in the spring of 2012, they really didn't look bad. So far this 2013 Aerolite
one we have 200+ nights going on it's 5th season.