Not knowing much about motorhomes and the fact this 1993 allegro bay 27.5 class c motorhome was free , i will ask this question. Before i got this rv there was a water leak on the left side in the bun...
Use the forum search feature - s/b a few lengthy threads with pictures on guys who had to re-build their cabovers ... should give you a feel for what's involved. Are you sure that the interior wood is rotted and not just the thin veneer stuff that covers the interior wall -- the later is easily replaced.