I had that problem on a previous coach. Although the four screws holding the mirror to the wall appeared screwed in tight, the real problem was elsewhere. Turned out that water intrusion had occurred into the screwholes and the integrity of the wall was affected. So the wall was actually flexing from wind pushing on the mirror. On that coach I was able to cut a 1"×4" board which I held in place inside, under the dash as I secured the new, longer screws from the outside of the mirror all the way through the sidewall into the 1"×4". BTW- I resealed all the possible areas where water could enter before making that sandwich with the wall in the middle. Never had another problem with water intrusion nor shaking mirror syndrome.
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