I just so happened to be moving 200 feet of pipe this morning, so you get a pic.
Stored outside for the last 16 years, sometimes in my gravel parking lot at the shop, but most of its life, on the grass in my side yard. ( I think it spent one winter indoors with my blown up 911 on it). Still not showing any signifigant rust. I think for the most part, trailers sit higher up off the ground so they don't rust as fast as a car sitting in grass. Also, they are heavy gauge steel, not thin sheet metal like a car.
I have been putting a tarp loosely over the deck, just to hamper the leaves , and moss gathering on it , it turns green if I don't do this. I pressure wash it every spring, and every few years, it gets a quick and dirty spray can paint job.
I love those trailex trailers. Nice grab.