I'm really surprised it leaks when extended. Does it make its way into your coach while extended or bring water in when retracting the slide?
If it wasn't so new I'd suggest your seals are dry and need to be lubed so they flip properly when extending the slide.
A simple solution would be to auto level your coach then raise the side opposite the long slide slightly to allow the water to easily run off. I actually have my auto level adjusted for a slight tilt to allow my toilet to flow better.
Hope this helps.
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