Some mineral stains are harder than others. I would try vinegar and rubbing, after washing to get all other dirt off. Don't want to grind it in.
Be sure to rinse it good and not let it stand on aluminum or other metal or the paint, if that is what is on the fiberglass or gelcoat.
I had a dickens ,of a time, getting it, off the side, of our MH when, the plug on the water heater decide to leak.
Might ask Mfg first off.
I hope someone here has a product, in mind that works better and easier. Keeping our rigs clean is a chore.
