Jack- here is a long shot question? In what proximity is the holding or retaining brace for your leaf springs? The reason I ask, since you find no moisture damage is that the coach may have suffered a very bad part of the road- pothole and that part may have failed.
On a side note, I inspected a coach last year, with a tag axle. The marble tile in the bath had a crack running the width of the coach that I could stick my tip of my pinkie in. Thus- over 1/8 in width. Can you take some photos of the coach from the exterior? We might see a deviation in your outside skin. Although, we can't help get it back to factory, we might be able to ascertain what may have happened. Thanks, Ernie