Very open ended so here you go:
It might be bad window seals. Silicone or having it pulled and resealed professionally.
Could be the actual frame against the coach. Again, either top seal or remove and do it.
Never said if it was a dual pane window or single. That matters.
It may not be your window. Could be coming from higher up. Possibly the front seam or the roof to sidewall seam.
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