There is a lot of information online, but I wanted this groups opinion:
RV Fiberglass Vs Aluminum Siding (Which Is Best?)
These are not the only choices. Many rigs have plastic skins, often bonded to structural foam, among other choices. LD's have primarily aluminum skins, which can be and are painted, a durable finish that will not deteriorate as fast as plastic and fiberglass surfaces. Also, the metal forms a protective conductive surface if struck by lightning. There is also fiberglass and ABS plastic used for various external areas, wherever those materials work best.
Steve