Most likely the floor is rotted or at least started to rot and will have to be repaired by pulling the slide out out of the coach and a new floor built and installed. You may also find that the main floor could also be rotted if enough water over time settled in on it. Unfortunately, this is not an easy or cheap repair. Good luck, stay safe and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7