The floor is made of 3/4" OSB or strand board. Once wet it turns to mush and needs to be replaced.
You don't say if you are doing the replacement or having someone do it for you.
If you are doing it, use 5 or 7 layer plywood (available @ Homedepot, etc.) Treat it with water seal then put it down. Make sure you seal the cut e dges of the OSB as once the seal is broken it will get wet again.
When you take out the old OSB you will notice a steel or aluminum framework supporting the floor. You will need to attach (screw) the plywood to the framework for support.
So plan to take out the OSB back to a supporting frame. If not possible the plywood will cantilever short distances and still stay rigid.
Once attached, get floor levelling material (Home Depot, etc) and apply. Some one makes a paste floor levelling material. Everyone else makes the material using water, which I did not want to do.