Once you start researching boat building, all sorts of intriguing possibilities pop up.
If you're really into pricey fiberglass trailers, you can get a current one from a B.C./Chilliwack manufacturer. An actual 5th-wheel too, with 3900# weight. For a cool 40 grand $.
http://escapetrailer.com/trailers/the-5-0-escape/
From factory videos though, it looks like like a straight shell construction - no balsa or foam core laminated composite. So either relatively flimsy due to a single thin shell like all the other fiberglass clamshells, or ungodly heavy like early f/b boats due to a thick layered buildup of fiberglass. Obviously the former.
For 40 thousand dollars though, I could probably make a really nice custom aluminum-framed 20-foot tandem toyhauler with composite shell. For an additional 10 grand, probably a 24-foot tandem gooseneck with comp shell, well within GVCW of my present truck.