The stone gaurds of which you speak are indeed made from stainless steel which does not need a coating. The black you are finding is most likely road grime. When stainless steel is polished, it does not turn black on your rag as does aluminum. Stainless steel sink cleaner is a very good agent to clean and shine your stone gaurds with. Even some of your powdered cleansers will work, although, if you use that type cleaner, you need to work with the grain. You also can use a fairly stiff brush on these.
Oh, the trim around the entire unit is made from aluminum. That may be where your black, and streaking was coming from. By the way, probably the best aluminum polish for RAW aluminum, (NOT THE SIDES OF YOUR TRAILER!) is Alcoa Aluminum Wheel Polish. You could use that on the edges of the stone gaurds.
Enjoy that late model unit!
2000 390 Landyacht XL
1989 29' Excella
WBCCI #6673 jerry Hodge
Have no intention of arriving at the grave safely, but rather to skid in sideways, chocolate in one hand,throttle in the other, totally worn out and screaming
"WOO HOO, WHAT A RIDE!"