I have no slide toppers and used to live on the coast in the Pacific North Wet. As mentioned, lower or raise the front of your trailer and pooling water will run off quickly. It will still be wet up there, but really, who cares? It never caused a problem for our trailer because, as mentioned, it will dry.
Quite honestly, there were many, many times in the PNW where it was raining when packing up and absolutely no way it would be dry before bringing in the slide. Sideways rain meant slide toppers wouldn't keep the rain off either. It will dry and I wouldn't stress one bit on this.