Your Greyhawk has a fiberglass roof like mine(same year and model). In the Spring, all I do is first rinse it down to get the heavy dirt off it, then get a bucket and put some automotive shampoo(Maguires) in it. You can also use Dawn dish soap. Get yourself one of those soft bristle brushes on a stick, the kind that a hose screws in the end. Then I start at the front and wash it like I am mopping a floor. Do about 4-5 feet, then rinse with a hose. Before you do it, carefully inspect each caulked joint for cracks etc. With a 31FS, never step anywhere close to the edge of the roof or you can crack it. Don't step anywhere beyond 5 inches in from the rounded edge.