On our 2003 Gulf Stream Yellowstone DP, we had the rubber flap w/Gulf Stream logo that was factory installed, On our 2004 Gulf Stream Yellowstone DP, we have the stone guard/flap that is metal on top w/a rubber flap that is hinged w/GS logo that was factory installed.
Neither one kept/keeps the back of the coach clean BUT they do help keep stones and debris from flying back hitting the toad.
The rear of the coach creates a big vacuum and turbulence that seems to suck the dirt and water onto the rear of the coach. I don't believe there is a way to stop this other than not driving when the road is wet, dirty, dusty, or driving only on a sunny day.
On our last trip from Florida the last of April was the first time we ever were able to travel a complete trip without getting into some rain and getting the coach filthy dirty outside, BUT there was still some dirt/dust on the rear of the coach after the 1,000 mile trip back to our Ohio home.