As long as the metal is sealed from air/moisture, it will be rustproof. Thin, factory spray paint may have microscopic "holes" which will eventually allow air/moisture to reach the metal and rust will begin to form. You see that on the edges of some metal pieces where the paint is the thinnest. Powder coating is considerably thicker and would be a better shield, but any breaking of the coating from, say, tightening screws or bolts, will begin the rust process.
The suggestions of mounting and then covering with Dicor is the simplest and very effective as the entire plate and bolts/screws are protected from the weather. Of course, you could make a plate stainless steel if you don't want the plate to be covered with Dicor.
Edit: You posted as I typed. Can the plate be covered with Dicor? If not, then powder coating or electroplating is the option. Since it is a magnetic mount, it will be important the powder coat does not weaken the magnetic bond. Stainless steel also tends to have a weaker magnetic bond. I assume it is a very powerful magnet to hold position on a moving vehicle, however.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.