DesertHawk mentioned a Scamp - there are a number of fiberglass trailers in the 18' - 20' range including 5th wheels available from a number of manufacturers. I have an Escape 17B with high lift axle that can go just about anywhere my tow vehicle can pull it.
Escape Trailer Industries makes a 15', 17', 19', and 21' as well as a 20.5' 5th wheel.
Again, there are many manufacturers of lightweight fiberglass trailers. Because of the construction methods, most fiberglass trailers will hold up to rough roads better than stick built trailers. They are also usually lighter, and more aerodynamic, providing better gas mileage.
I have links to blogs by owners, manufacturers and other information at my
Fiberglass Trailer Page.