What's in between the floor and underside of TT?

The fabric material is Darco. A Google search will bring up suppliers.

We had the same issue with out TT. We had a roof leak and all of the water collected in the underbelly. There was a ton of water and wet fiberglass insulation in there. I would recommend cutting some slits to see if it drains. I removed all the Darco and insulation to let it dry and will be replacing it with foam board insulation covered with coroplast. Whatever you do, don't let the wet insulation sit in there.