Black material on top of frame/under floor

I believe the trade name for it is Homasote. It's an insulating fiberboard, at least in building construction. You can still buy it at Home Depot and lumberyards and comes in brown or a black coating. We used to use the black coated stuff under house siding and stucco. It has some reasonable resistance to moisture, at least the black stuff does.

My trailer never had it, so I don't have any experience with it or how it was originally installed, and why some manufacturers used it abd others skipped it. Is it sandwiched between the floor and frame? It would be easy with the frame off I would think.
Whether it should be replaced and how to do it, I'll leave that to someone else to respond.

I believe that Homasote can be soaked in petroleum, to make it impervious to mold growth and deflect most organic matter that would confront it from the road or ground. It also made it last a long long time. It was in very good condition on my Mobile Scout before the rebuild… it was the plywood on top of it that was in utter shambles! If your trailer is constructed like most, the side wall sandwiches are side bolted to lumber resting on the outer edge of your steel trailer frame. So, theoretically you could replace your floor layers without removing the walls. But I would suspect that if your entire floor needs to be replaced, your wooden edge framing is also not in good condition… which means the walls need to come down too.  Also, if you were planning on removing your interior built in furnishings (base cabinets, wardrobe) to access the entire floor plate… your trailer will require bracing as to perfectly preserve the dimensional shape, so that all those furnishings can go back in and actually fit.  Consult Mobile Scout's videos to completely understand the order in which components are assembled/disassembled. The floor materials used in my rebuild were, in this order: lumber on the steel frame (some clad in sheet metal… further protects wood/makes a good terminate shield), galvanized sheet metal (facing the road), 3/4" marine plywood, 1/4" oriented strand board (board joints filled with Bondo, sanded), Marmoleum sheet glued in place as finished floor.  

Celotex was the original product. I like it and try to save it whenever possible.

Chametzoo, you are too funny...