I think you're referring to your Slide awning/topper. Ours is also long and can do the same thing, except that it is on the driver side.
I would suggest added a piece of foam, or blanket, over the door end of the Slide, stuffed in between the top of the slide and the awning. It will probably keep the water from cascading over this end of the awning.
Just remember to remove this when running in the slide!
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