Had this issue on a landscape trailer one and found that hitting them with some spray can clear coat after re-painting (or while the wheel is still new) helps keeping the rust from forming.
Many RVs are manufactured in Northern Indiana. Many are then are towed out to the dealers in the winter months for the Spring rush when salt is on the roads. This therefore leads to rusting on unwashed parts including spare tires. Surface rust has nothing to do with the integrity of the spare. Clean it up and repaint it or just replace it.
I'd be careful spraying WD40 on things you intend to paint. It will prevent the new paint from sticking to the metal. Just make sure you clean it with plenty of soapy water and rinse it well before you repaint. Just my 2 cents (FWIW).
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2013 Eagle 266RKS
2011 Ford F-150 w/3.5L Ecoboost & H.D. Tow Package