What caused all of that rust so quick?
..... I have no idea why trailer manufacturers even use boxed/tubular steel frames any longer....
Tube construction has a high weight:strength ratio. Weight = $ in construction costs. C-channel is considerably heavier (more $) but with a little attention, will last forever 'cause there's no place to retain water. I've got one going on 30 years old and going nowhere near the salvage yard.