John "in Michigan" has shared some sage advice and direction, and the more you poke around the more you'll get a sense of how the components in your Trillium
work together, and how much restoration/repair is necessary vs cosmetic or personal preference.
If you become familiar with the frame and corresponding areas inside the trailer, this can give you some idea of which areas provide critical support. As John mentioned, the outer edge of plywood over the "pontoon" is less critical, does a good job of keeping your stored items out of the puddle of water the pontoon is supposed to collect and drain. Conversely, the plywood area around the bolts to the frame needs to be sound.
I am lucky to own a '79 4500, and have a wee patch of partial thickness soft plywood in the same area. Both rear storage areas were lined with pieces of Ensolite and foam sheets like you'd pack your plates in. Really impaired ventilation. Your photo makes it look as though some square item sat in your storage area and held moisture against the plywood, in that case the soft area may be localized and require more patch/treatment than removal.
As an alternative to replacement, some owners treat wood with has small, or thin, or non-structural defects, with epoxy and or wood hardeners.
If your water tank is under the floor, at the rear of the trailer, you may be able to lift off the cover and, with some contortions, see the edge of the storage area plywood you have questions about. This inspection might allow for further evaluation. Will add, just in case, if you take the water tank out, DO NOT STEP IN THAT COMPARTMENT! Pretty likely you will crack your shell, and possibly step through.
Lastly here's a screenshot of a current 1300(?) build. Might help visualize how they are put together.
Hope you enjoy your trailer.