I took the corner apart and peeled up the siding to expose the rotten 1x2 that held the whole corner up.
I re-fit a 1x6pt back into that side corner attached to a still strong original 2x3, cut the studs up to bear on the 1x6. Got the whole corner back to life!
I am playing around with a design idea to retrofit a smal A/C unit into this upper cabinet over the rear side bed. Stay Tuned!