In my Monaco owners manual it says to retract the slides during a heavy rain.
I don't think you problem is with the topper collecting water. I think it is with the topper not being wide enough to keep water off of the top of your slide or from rain blowing in the sides and collecting on the top of your slide.
if you look at the topper as it operates, all of it remains outside of your coach so any water on it remains outside. Any water getting inside is coming from water that has gotten on top of your slide roof that the weather seal slide wiper is not pushing off as it retracts.