I been using a "paint" product called POR-15 for years on my car and motorcycle restoration projects and found that it also works great on my motorhome. POR-15 is a special paint that brushes on like paint but dries to a powder coating like appearance and hardness. It's great for painting metal frames, welds, or anywhere you may get moisture. You can paint over rust as long as you get wire brush away the loose rust. The paint dries quickly once exposed to air and the instructions tell you that once you open a can, you cannot reuse the leftover as it will dry out in the can. POR-15 is sold in different quanities and the hot tip is to buy the "Six Pack" of 4 oz. cans. Six 4 oz. cans does not sound like much but a little goes a long way. Even though I have a new motorhome, I've been painting and touching up the frame and welds where the manufacturer did not do a good job of getting paint on everything underneath. I painted the whole frame and tray underneath the batteries and it looks great, almost like plastic coating. Anytime I see a little rust start, I paint that area. Everything will stay looking new for years to come. You can get POR-15 paint at
http://www.por15store.com 
P.S. Use disposable gloves when you use this paint and if you going to be painting underneath your motorhome, I recommend a shower cap. This paint will not come out of your hair if you head touches wet paint. Believe me, I speak from experience!