Have you looked underneath the chassis in that area for a missing/lose cross member or brace of some type? Did this just start? Have you had water issues in the recent past? It could just be as simple as an area that didn't get supported well and has just grown weak over time. You might be able to insert some type of brace or cross member in that area. Previous truck camper a few years ago did similar thing. Found the cross member, which was untreated 2X4, had come lose from the staples they used to install it. Staples rusted and it gave way.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie