17 bottom consists of a fiberglass outer shell resting on the frame, then a 5/8" plywood layer above that. On the top surface of the plywood layer, it is bonded to/tabbed to the side of the fiberglass outer shell using more fiberglass. This fiberglass applied to the top surface of the plywood layer extends a foot or two in from the outer edge.
4500 is of similar construction.
Keep in mind that the plywood floor provides structural support to portions of the fiberglass body (bottom, sides and roof) that rest outside the perimeter of the frame. Without the plywood support, the fiberglass will sag over time. To complicate matters, the plywood extending outside the perimeter of the frame is cantilevered. So if there is extensive rot extending outside the frame, the repaired layer must be attached to the solid layer inside the frame to form a single rigid layer that is at least as strong as the original supporting layer.