We had the same thing happen to us several years ago. We do, however, carry more than the average user since we fulltime and run a business. DH dropped the belly and used several pieces of 2"angle iron to push the floor back up into place and support the cargo bay floor...it was placed on top to the drop frame I beam and runs from side to side...we could now park and Elephant in the bay.
DH screwed the angle iron into the bottom of the cargo bay floor where the furring strips run, under the black vapor barrier fabric, front to back...have never had anymore problem. Thought we had pics of the fix, but only found pics of the issue. Hope this helps...