Best ways to protect trailer's under carriage?

Don't worry about the underside to much, check it over once a year or two and clean up any bad spots. There are 100,000's of trailers on the roads that get pulled through everything on the road year around and they're still going strong. You have to remember that you're asking the elite RV guys on this forum and they go way above and beyond what most folks do. Guys on this forum take RV's as a hobby instead of a camping tool. This isn't a bad thing but sometimes things get carried away like don't drive on salted roads. If I did this I would only be able to use my trailer 5 months out of the year as they don't call the Great Salt Lake the Great Salt Lake because it's shaped like a salt shaker.