Class C body sag

The front passenger side of RV body has started to sag. I thought maybe the screws that go through the exterior body wall to the floors have bent. So I jacked up the body shell and replaced the screws with larger. As I lowered the body down It sagged and I now know it's the floor that's causing the sag. I not sure how to go from here. Add a support under the floor underneath? The picture shows the stripes not lined up from the sag.