I think you'll find that the corrugated plastic (Coroplast) is just hanging there to hold a few pieces of fiberglass insulation to the bottom of the trailer. The cross piece might be nothing more than a place to screw two different pieces of the Coroplast into.
At any rate it's not structural so go ahead and pull it down and have a look. It does help on reassembly though to use something to hold the center of the sheet up while you install the screws around the edges.
another post dealing with the underbelly
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