I bet you never gave a thought about your seams and seals and assume that your units was water tight and it may have been when new, but seals and caulk crack and fail without regular maintenance and now you have a water intrusion issue and I bet it's a lot worse than just what you see (screws pulling out of rotted wood under the outer skin).
Couple choices, rip out the interior walls around the problem area (I bet you'll find a lot more than just that area is rotted and repair it, get it professionally remediated (very expensive) or sell it and get what you can out of it, being honest with any buyer about the intrusion / rot issue.
There is no easy way to address the issue and any 'band aid' approach will only be a temporary fix because rot is progressive, once it starts (mold spores digesting the lignun in the wood, it keeps going and it's obvious to me that it's wet inside the walls.
If you aren't flush and cannot afford professional remediation or you aren't handy with wood working and structural member replacement, I'd sell it.
Bet on your next unit, you'll pay attention to the seal maintenance.