Not dry rot, it's wet rot from water intrusion from failed seals because the previous owner failed to maintain the seals (like probably 99% of owners do. They assume (wrongly) that seals (especially in the front and on the roof, never need maintaining.
Unless you are extremely proficient in wood working and have lots of time to repair it, I'd loose the unit ASAP. There are shops that do it, but it's very expensive and 'Git Rot' which is a marine product, won't help you, it's for small areas. You have structural rot which needs to be torn out and replaced and, if the wallboard inside was compromised, it's bad.
Dump it is my suggestion.
