An example of a smooth (non corrugated) aluminum would be the Livin' Lite Camplite trailers. The smooth aluminum is attached to aluminum studs. No wood products whatsoever, so water intrusion should not be able to cause the extensive damage one can see with other construction types.
Another trailer type not yet mentioned is the molded fiberglass (often called "egg") trailer. Scamp, Casita, Parkliner, and Oliver are examples. Such construction minimizes potential water entry points by eliminating roof-to-sidewall and sidewall-to-sidewall seams.