While the actual manufacturing process used to make molded fiberglass probably doesn't qualify as "green," the end result is compatible with green values. First, as mentioned, their small size and light weight
means less fuel used to pull them around (reduce). Second, they last so long that they are typically handed down through many owners (reuse).
Most molded fiberglass manufacturers sell direct-from-factory, so no, you can't just walk into a dealer and look at one. They offer something better (though perhaps not as convenient): they will refer you to an actual owner near you who will show you their trailer. Beats a salesman in my book. You can also visit a fiberglass egg rally
and look at many different makes and models in one place. Many have a time set aside for "open house" tours.
Regardless of whether you look for a fiberglass trailer or something else, beware of manufacturers' dry weights. Actual loaded weights can be much higher. See the thread "Trailer Weights in the Real World" in the General Chat section of the forum for more realistic data on molded fiberglass models. That, along with the towing information in your vehicle's owner's manual, will get you started in the right direction.
Best wishes in your search!
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