I hear ya! I live 4 blocks from the Pacific Ocean. With the onshore breeze salting everything up, there's nothing around here that doesn't rust. Knowing that and dealing with it with other trailer frames, when we bought our RP175 3 years ago, I had the frame undercoated. That has held up well, but I'm still sanding and repainting the tongue, steps, leveling jacks, and spare tire mount all of the time. I asked here the same question you just did, and everyone recommended doing what I've been doing with Rustoleum. Finished all but the steps last week, and it's looking pretty good again.
I've had to crawl under other trailers to sand and refinish the frame. It is a real pain. So if you, like me, know there's going to be a problem, then I do recommend that you think about undercoating.