There is what feels like a "soft spot" in the floor of my trailer. It is the only spot in the floor that flexes slightly as I step on it. It is an unsupported seam where 2 pieces of 3/4" plywood flooring come together.
The seam crosses over the heat duct which runs down the center of the trailer. The duct is stapled directly to the underside of plywood flooring. It does make the center isle warm for the feet when getting up in the morning or getting up to go to the bathroom at the back of the trailer when winter camping.