Lance Sagging Cabover

I used to have a 98 Skyline weekender and had a similar issue. My framework was made out of wood with metal flat brackets stapled in the corners. There were cracks in the main frame supports allowing the weight of the structure to make it sag. After removing some corrugated siding and saw what the problem was I closed it back up and sold it to a guy who wanted to have it parked on a flat pad and not even use it on a vehicle. The siding on yours might be what is giving it some strength to keep it from sagging more.
Don't let my story scare you mine was worst case. Try to investigate and see the cause of the problem and go from there. Good luck.