Lots of info on the forum about this issue. Depending on type/brand of topper, you can adjust tension. Basically involves removing rivets or screws at end of topper roller and adding more tension using a screw-driver as a pry bar to hold spring in place. Sort of like setting tension on a garage door--though not as dangerous or scary!!!!
Overall problem with most brands of topper, especially those mounted on long slides, is that they allow the middle of the topper roller to sag in toward the coach. This allows topper fabric to gather more and more water as roller and fabric continue to sag under the weight. Generally speaking, adding more tension to the roller doesn't keep the middle section from sagging. Most solutions involve putting things like swimming noodles, balls or other objects under the topper fabric to eliminate the mid-point sag.
PS--A contributing issue with the toppers on most Alpines is that the geometry between the top of the slide and the roof line doesn't allow for enough slope to get the water to shed rather than pool.
2003 40' MDTS
Garden Ridge, Texas