We were told when we bought our RV that because the sink was not stainless, it would stain from coffee and other things. Within a year ours was looking pretty disgusting. We had tried many products to clean it and nothing worked.
We got to the point where we were talking about getting a new sink. We knew it would be costly but we seemed to have no choice.
One day I went to a store and bought Soft Scrub. I knew it would not damage the sink surface and it was the one product I had not tried previously. Well, it worked!! The stains were gone and the sink looks brand new again.
So if you encounter this problem, there is an inexpensive fix.