Rubber or vinyl is used on 99% of towables because it can flex with the structure going down the road.
Fiberglass and aluminum doesn't give as much. The Work and Plays are built very rigid, more like a utility trailer than an RV. Motorhomes also use fiberglass, but again, are a much more rigid frame and don't flex as much.
Old trailers (think '70's) used seamed aluminum, and they expand and contract with changing weather, and you could never keep them sealed.
Steve and Vicki
2011 Dodge 3500 CTD Outdoorsman
2013 Columbus 320RS-ITS HERE!!