Actually quite a few boats are molded of solid fiberglass below the waterline. A very few solid glass everywhere. To achieve the stiffness of a sandwich panel in solid glass weighs and costs a lot. The reason not to use it below the waterline is that ANY penetration results in water in the core which can lead to problems. Many poorly built sandwich boats in the 70s and 80s caused buyers to begin shying away from the concept. Certainly all light weight racing boats are cored throughout as it is much lighter.
While water in the core in a camper is of little concern, the price point of campers does not support high quality cored construction, we are left for the most part with what Bigfoot and Northern Lite do. Earthroamer and Bahn are examples of true cored camper shell construction, and have a price that reflects that.
