In the past I've tried a number of products on the rub rails to keep them looking black, including 303 protectant and BlackWow trim restorer, but nothing lasted. After a few weeks they'd look gray and weathered again. My coach spends about 3-4 months outside and the rest of the time in indoor storage. We are based in Massachusetts and camp about once a month in the New England area, and we do a month-long trip to Florida each winter. The coach gets washed a few times each season with Turtle Wax Hybrid Solutions Ceramic Wash & Wax but I'm not super attentive to it.
Last year at this time I tried a product called CarPro DLUX. It ain't cheap but it has performed beautifully. Here's a photo from a few days ago:

Needless to say, I highly recommend this product.
Last year at this time I tried a product called CarPro DLUX. It ain't cheap but it has performed beautifully. Here's a photo from a few days ago:

Needless to say, I highly recommend this product.