There used to be a product called an "air wedge" --- it was an inflatable, wedge shaped, bladder that slides between the topper awning and the top of the slide out (about $50). It allowed easy rain and debris run off. Only drawback was forgetting to remove it prior to slide retraction
I can't find the internet site anymore.
I'll do a little more research -------- I have one stored away someplace in my rig.
2002 Newmar Mountain Aire, 37', 3778, W-22, 8.1 Vortac, Ultra Power upgrade, CAI (cold air intake), Taylor wires, colder plugs, Koni shocks.