If you only get the leak with the slide in my guess is the face trim on the low part of the slide needs caulking.
I had the same issue on my Miramar and after spending days with a garden hose finally discovered the entire length of the slide had no caulking on the top of that lower trim. Most of the time the water that would leak in would evaporate on the floor before I saw it. I actually discovered the leak by sleeping in the RV in a truck stop during a thunderstorm, with the slides in. I got up when a garbage can blew past the RV and walking through to the front I stepped into a small puddle of water.
I spent the next several days discovering and solving the problem.
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