You will want to investigate the extent but also the source of the rot. In the case of my
Trillium, there was some rot but only immediately around the screw holes for the straps that held down the tank. Easy fix. Had it been more expensive I'd have replaced the plywood in the area affected and recoated it with resin. It's important to keep this area of the floor strong because in my case the bolts that hold the body to the frame pass through it.