I did, and I did the same thing on the other side. My thinking is that as you apply vertical load at the front of the cabover, that wide piece is going to be in compression, not tension, as is that diagonal piece that comes into the base of the wide piece.
The top of the wide piece that was not rotted is very solid and glued tight. As long as it has a solidly glued piece to push against it should be OK. Gluing the new piece to the inside paneling and then gluing the outside to the whole thing should give it the lateral stability is needs and used to have.
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This Lance also had a piece of aluminum flashing running between that large vertical piece and the sheathing. Was too soft and flimsy to be structural so I didn't replace it.
