I think a lot of them are acrylic so it could be that. I would not use anything abrasive on it, including Soft Scrub and Mr. Clean erasers. The stove cleaner (Ceramabryte) I use to clean my flat top stove at home actually works on my porcelain kitchen sink at home without scratching it. The Soft Scrub scratched my porcelain sink terribly and I will never use that stuff again. I wouldn't have a problem trying the Ceramabryte on my acrylic trailer sink. Your cleaner needs to be totally non-abrasive.
Could try something like Formula 409, Kaboom or a non-ammonia based glass cleaner.
I just read a list of cleaners that should NOT be used on an acrylic sink. Soft Scrub is on that list.
You could try some vinegar. I would use it straight. If all else fails, try filling the sink to above where the stains are with hot water and add a tiny bit (capful) of bleach to the hot water and let it sit for a bit. Or try the Oxiclean. Just don't use anything that says "scour" and don't use an abrasive cleaning tool, just a cloth.