Possibly you could weld up your 14" X14" square tube outside the coach and then ad gussets and brackets to it so when you install it you could just bolt it into place.
It wouldn't be nearly as strong as welding it in place, but then I saw one Flx where the cut out the longitudinal member and boxed it in with wood 2X2's. t had been going down the road that way for years with no problem.
I personally wouldn't do that, but it is a thought.
By the way, I still haven't found the box of lightbulbs I told you about.