Myco also pressurizes their trailer frames. Not sure if they do 'em all this way nor do I remember what they fill them with. With them, no corners are cut though there are a number of high quality trailer manufacturers out there.

Never heard of using Argon but it makes perfect sense to displace the oxygen. No oxygen equals no oxidation which equals no rust. Same reason why argon is such a good shielding gas for welding.

I believe it was Poker Runs America that recently had an article on custom Myco trailers. Pressurizing keeps the water out and the guage mounted on one of the frame rails confirms whether the frame is airtight. They too probably used Argon. For what it's worth.