We just sold our 17' Casita, and it had the Dexter torsion axel. We had the hi-lift option for 3" extra travel/clearance and I'm sold on that type axel. Over 15K mi. in 2 yrs. There was a lot of discussion on the Casitaclub.com site re: torsion axels but those archives are not yet available on their new forum.
Casitas are totally fiberglass, even the bottom, one piece with a center seam that is glassed in. The black & grey tanks are exposed & not ducted, but the fresh water is enclosed. We had a lot of fun with it,...but now it is time to matriculate to A/S