Our Springdale TT came to us with a considerable amount of rust on the frame and joints and such. One of first things I did was to "try" to brush and repaint. Well, the brushing became a real pain in the neck real fast, laying on my back on the ground ... didn't I give up laying on the ground when I gave up tent camping?... anyway, I finally just went right for the paint. I used black Rustolium metal "oil based" rust inhibitor spray paint (about 10 cans of the stuff) .. wasn't cheap .. getting that much at once ... and took to spraying everything I could see under the trailer. Worked great!
Next year I crawled completely under the trailer again and inspected (early spring) like I always do. I then bought about 3 cans of the stuff and sprayed the spots I obviously missed the year before.
Well, 6 years later (3 weeks ago) we traded for our new Outback. Before trading, I DID crawl under the trailer, and poked around with a poker, did a little scraping here and there, and had another spray can handy. Except for the areas where I poked, after 6 years, no rust, and the paint was still holding fast. There were a few places where rocks and such had hit on the frame, you could tell by the chips, but no apparent rust. What little was there was just surface. So the Rustolium held pretty good, and the underside of the trailer STILL looked real good!
Rustolium (the oil based version) has a rust inhibitor that actually penetrates and stops rust. You don't need anything else. Shop wisely, and don't be afraid to spend an extra dollar a can. Considering your camper probably cost over $20,000, 5 extra dollars to give your camper 6-8 years of trouble free service is well worth it!