Welcome, Jack! You’ve come to the right place for small, lightweight, all-molded fiberglass trailers and fifth wheels.
Since you mentioned the rails, we’ll start there. Scamp
both make molded fiberglass fifth wheels (technically Scamp’s is a gooseneck, but their in-bed ball mount uses the same rails). There are also vintage Bigfoot
fifth wheels to be found, along with a few other very rare vintage molded 5-ers.
They’re all smaller than the typical framed and laminated Elkhart fare and all are half ton towable.
Best wishes in your RV journey, wherever it leads!