"This thread has been quite useful as far as Showhauler frames, but what about NRC or Haulmark or Powerhouse or any of the companies that use aluminum frames?"

I toured the NRC factory in May 2004. At that time the only frame construction that I saw was welded steel. Same as Showhauler and United Specialities.

Haulmark uses a aluminum frame, vacuum bonded, similar to many of the motorhome manufacturers. Interesting sidenote I was told my a Haulmark dealer and saw one trailer that haulmark is now using FRP in their large stacker race trailers. The other construction style would not hold up to the stress in the tall stacker trailers.