There is a product sold at Lowes, Home Depot, Walmart and some grocery stores called CLR .
I have had good luck removing rust stains with it.
You may want to follow that up with some rubbing compound and a good coat of wax.
You will also have to find the source of the rust and remove it or the same thing is going to happen again.
I agree with other posters that it is most likely a random screw that is not stainless steel . You could take the cover off of the a/c unit and have a look in there to be sure, some Thor employee may have left an old screwdriver in there .
I found a pair of pliers and a screwdriver in my Vegas !
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