Might not be your problem, but I had a rusty looking thing below a screw. Pulled screw and found that is was zinc coated and they are not as good as stainless and zinc does not last very long compared to other metals IMO.. They lose their zinc coating and start rusting IMO.
So screws and hinges and any other zinc exposed to weather, I've replaced with stainless. Others I couldn't match up I got new zinc screws and others I couldn't find replacement, I put a clear coat on them, (better known as wives clear finger nail polish) seals up rusting zinc pretty good for awhile. Same goes for cheap zinc rivets, replace them.
To get off my brown spots I picked up some "fiberglass rubbing compound" and very gently used a Q-tip and removed brown spots and put about 3 layers of wax over top of them. Or better yet stop by a fancy boat place and ask. How do I rub out my finish? Mine was on filon siding. Whatever you do don't be aggressive with finish, slow and easy is the key IMO. Of course if brown spots are too deep I'm not sure what to do. Finishes are only so deep and measured in 1,000 of inches so be careful.
All of my brown spots didn't really sink into my head until one day picking up mail and noticed how the mail box was getting rusty on top. RV metal is also exposed to the elements and some metal loses its protective coating just by being exposed. And maybe that's why they make anodes out of zinc also.
2003 Majestic 23P... Northwest travel machine
2013 Arctic Fox 25W... Wife "doll house" for longer snowbird trips
2001 "The Mighty Dodge"... tow vehicle for "doll house"