How far do you want to go?
Scroll up in this forum to the Truck Camper University sticky thread, open it and scroll down to "Camper Structural: Repairs". Look at some of the threads presented there to get an idea of what may be lurking behind the plywood.
It appears to be always worse than it looks and more time-consuming to repair than anticipated. Repair is fairly simple and doesn't require particularly exoctic tools or materials. But it takes time, during which you will not go camping, and a dry place where you can store the camper while you work on it.
Patch it up with as little effort as possible and make it work for another season or two, or tear into it, knowing that this may snowball into a very involved project.