The whole flip up is shown as one part number in the manual:
141784-10-02A EA COUNTERTOP EXTENSION - 1/4"VENDURA - ALMONDINE
I'm sure Lichtsinn or Winnebago Parts can give you a price.
Winnebago themsleves can e-mail you the drawings of the sink cabinet. There may some reinforcements added in 2006 for the flip up. I'm sure it wouldn't be hard to duplicate what they did, if they did anything.