I've been thinking about this steel and rust question, and I don't recall any rust in my Cargo Craft of Georgia brand trailer. They have a plant in Texas as well, and at least one distributor in Colorado. (Might as well post the link), they had one of Cargo Craft's custom off-road trailers that I really liked:
http://www.coloradotrailersinc.com/2016-cargo-craft-elite-v-5x10-off-road-cargo-trailer-new-cargo-trailer-co-i1821272In their older literature and webpage, they bragged about the frames being completely painted before assembly, and my main (bottom) trailer frame was coated in some thick black rough texture stuff that is still in place. They also use 1" square tubing in the walls and ceiling as opposed to z channel etc. and I'm pretty sure I remember everything was painted.
They also had a water test rig, and every trailer went through to ensure no leaks. I was a little disappointed to find most of the old name brand trailers I was familiar with had closed their doors, and were now just a brand owned by a large conglomerate, so I took my chances with a less known, smaller company. Had a few minor issues, mostly with communication between dealer and factory on some custom requests by me. They sent a factory crew to my house to make the corrections. Mine is basically their Expedition rig, but I chose straight axles/leaf springs for more ground clearance.
http://www.cargocraftinc.com/texas/expedition.phpInterior of mine:

Build thread:
http://www.tnttt.com/viewtopic.php?f=42&t=48345I'm sure there are much better trailers out there, but in terms of value, strength and quality for the dollar, I'd buy another one from them if I ever had to. Hey, I just made a commercial! (where's my check?!?)
Welcome, and good luck, whatever direction you go.