I've yet to hear about a frame component breaking, bending or failing. It always seems to be the joints (welds). So it is not a material problem (whether boxed, I-beam, 10-12-16" etc etc etc) it's a manufacturing problem. Even a lightweight/mass produced (i.e. Lippert) frame would benefit from proper gusseting and quality welds. I'd bet dollars to donuts that you could take the best/heaviest frame and screw it up with poor joinery.
Look at the aircraft industry. Light weight frames but properly engineered and assembled (not welded by the way)
As the bean counters take over the more and more companies (not just RV) and the bottom line becomes the most important issue two of the depts that are first to go are R&D and QA. They now depend on the SUPPLIER to do their "quality?" inspections. How screwed up is that!
BTW, your "foreign" car was probably built in the USA and you truck has a better than 50-50 chance of being built in Mexico or other "foreign": country.
Oh yeah, as an afterthought.....do you really think that Lippert has a special team of welders that only work on DRV frames? I doubt it!! It's just that there are a lot less DRV units out there than ALL the other frames they make. (and that may change shortly)