And...as in any wood rot situation...the suspect area will increase the more you tear into it. The cross members/trusses may have damage also, which will roll into the inner wall areas. Big job any way you look into it. Can you attach any pictures you have to the extent of the damage you see...before tearing into it? Have you been up top and stepped around to see how soft it may be and how big that soft area is?
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie