I imagine with that long of a slide topper the rain will indeed pool in the center, it's just too long not to. But so what? Ours is a little over 12 feet and the water pools some in the center. But we don't worry about it and don't do anything about it. The water either evaporates or gets dumped when the slide comes in. We simply don't do anything about it and it hasn't caused us any issues. I'm not going to put things under the topper simply to stop the water pooling, I bought the toppers to lessen the amount of work, not add to it.
I've seen where people have commented on this "problem" and still fail to see where exactly the problem lies. This is our second 5th wheel with toppers and the only thing to watch out for is to not stand near the slides when they are going in after a rain. :) You will get wet!