The full wall slide might be more susceptible for water to "Pool" on the topper, but it also occurs on shorter slides. After a rain, I just bring ours in 1/3 of the way and watch for water to drain. Then 2/3 for more drainage and then finally all the way in. We have never had water get inside with this method.
One of the nice touches by Newmar is where the topper recoil is mounted. Instead of mounting it to the outer edge of the slideout to be unwound as the slide goes out, it is mounted to the coach above the slide to be pulled out by the slide. This puts a slope to the topper that sheds the rain water as it falls and does not accumulate. Unfortunately, my Tiffin has the topper mounted to the slide out.
2013 43 QGP Allegro Bus
CJ7 Rock Crawler