I opened up the back corners, pealed the skin back and am amazed at how simply the trailers are built (saw the same with the ceiling repairs.) Took a wood chisel and hammer to the rotted wood and chiseled till I go to good wood or removed the whole piece (bottom left quarter panel)
Also - someone in the past had done a quick cheap repair with spray insulation foam. Got it all to a point of ready for repair, splicing and new. I sprayed all old and new exposed to the elements with flat black paint so that another layer of slight protection was added.
A lot of debris form the repairs.
Old wood replaced with new KD fir or cabinet grade plywood.
Views from inside. old and new joined by piece screwed on top.
Views from outside. added a 1/8" luan plywood to strengthen the back.
Put the skin back on moved onto removing the old, old butal tape at all corners and replacement.