Straight up if it's spongy it is trashed. You will have to pull back the flooring, remove and cut back to good wood, screw down new ply over the frame and then glue the floor back down. Trailers use chipboard for the floors which is even less durable than OSB. I do repairs with CDX.
While the floor is open use a coat hanger to reach all the insulation in the frame bay to remove it. It will be soaked with water. Let the bay dry out for a day or two before putting new insulation in and then fixing the floor as above.