I rarely post anymore but may be able to help so here goes.
Buddy had a new RV with a window that kept leaking. Like yours, nothing was obvious so we removed it and found the leak.
The aluminum frame is an exruded piece that's formed to fit. Where the two ends come together there will be some goop to seal it. If there's a pin hole, bubble or you can see any of the end of the aluminum, it will leak there.
All his needed was a dot of sealant to cover a tiny hole, which we could have done without even removing it.
So just look for where the two ends come together.