I find this topic to be interesting and controversial. Light duty trucks do use hydro-formed high strength steel boxed frames but all the medium to heavy duty trucks (including my little 1 1/2 ton Unimog) seem to use C channel that is bolted or riveted together. What am I missing ( are there any metallurgical/structural engineers reading this that can tell me the answer?). I figure there must be a reason why C channel is used in heavy duty applications. I also wonder why so few people galvanize their projects. There are 2 galvanizing plants within 45 minutes of my house. I have used the closer plant to galvanize a rack I built. There is a minimum charge and it is expensive but never having to sandblast and repaint a project is worth the price of admission.
There is more than one type of C-channel and I think the they are being used here in a confusing way. Capt Eddie is talking about structural C. I dont recall ever seeing any truck builders using it, at least not recently. They use open box channel, the material of which has a uniform thickness all the way around. The reason they use it is it makes it easy for upfitters to add stuff like flatbeds, dump bodies, utility boxes, etc. There is a builders code that the manufacturers adhere to so upfitters have a consistent and uniform platform to build off of.
Another reason is open box channel doesnt mind twisting some, which comes in handy when you're taking your truck off road across uneven terrain. Thats why your pickup truck bed isnt attached directly to the cab.
Structural C otoh doesnt really like twisting, which is one of the reasons it is recommened that it is bolted/welded up against something when used. Like another piece of C to form an I-beam. Another reason it's recommended to back it with something is you cant center a load on top of it.
For what the OP is doing I believe he is much better off using the box tubing and being done with it. Unless he is pulling his trailer through the ocean on a regular basis rust isnt going to be an issue. I find the notion of burst tubing due to frozen condensation...unlikely, and strength of weld a non-issue as long as the welder is competent. Welded properly you will destroy the surrounding base material before the weld fails. And yes, I'll take 4 walls of welded .120 2x2 over the .250 C-channel being proposed.
I have a bit of experience when it comes to joining metal. In my line of work .0625 mig is used for detail work. Real welding is done with .120 wire or bigger. A 1/4" stick electrode is tiny. On the last job that I was personally involved in I burned up more electrode than everybody on this website has, combined, for life. There's a chance I'm wrong, but unless they can add up all their usage and exceed 30,000lbs, I win. And compared to most of my welders I'm a light weight.