You didn't identify what model of National RV coach you are interested in, but I'm pretty it's a metal frame. My 2002 Dolphin LX was.
You may find a brochure that mentions the construction, but actual drawings were never released.
I found a brochure for the 1999 Tropical model - metal structure.
https://www.rvroundtable.com/wp-content/uploads/National/1999.NationalRVTropiCalBrochure.pdf