Our coach has the now-discontinued 'Summit' system that Carefree used for several years. This is an aluminum bar that raises as the slide is deployed which lifts the center of the topper. It doesn't work well since water may still pool on the topper half toward the coach wall.
As noted, slope (or lack of it) is the real culprit. Some very tall coaches have the slope. Most all of the Country Coach Magna's and the newer Newmar Mountain Aires, Essex, and King Aires have much better systems with good slope to the topper.