Is it Lance ?
Typical issue with them.
Even on the picture you can see steel chanel in the vertical corner and steel chanel under the slide. But those steel angles were never connected and there is limit how much wood can hold. No dryrot necessary.
If you can get welder there, all it takes to patch the frame is add about 2" of steel to connect the channels. That suppose to be strong enough to support the jack on empty camper.
Once you get it home - you can make it longer project.
The siding buckled at cabover pivoting point is "normal" as well.