For an older RV I would look at something like tractor paint from TS store. It is made for tractors that sit out side for years in the weather and sun. It is much cheaper than car paint or boat paints. The paint cost about $30/gal and from what I have read is that the only down side is that they will fade in the sun after a few years but will fade very gradual and evenly. I would prepare the surface as others have suggested then use a spray gun to apply the paint. I don't know if anyone one here has tried this type of paint but I would be interested in their experience with it. I talked to a farmer that told me he used it on all his outdoor equipment and it holds up well.