Slidetoppers are meant only to keep debris off the top of the slideroom, not to run off the water. They will pond with water though some have enough pitch on them to shed it. You do want to empty them, as the water will stretch and wrinkle the fabric. The easiest way is, as one other poster commented, to run the rooms in and back out. This empties the water off them, and you should "exercise" the slides weekly anyway. Balls and pool noodles under the toppers work, but are labor intensive to install and take off. Do NOT try and put more tension on the toppers. The spring is stiff and if you aren't experienced with awning springs, you could get hurt. Also, if theres too much tension on them, they cause the sliderooms to not extend and retract squarely, and in extreme cases, not seal properly. Hope this helps.
1997 National Seabreeze 133 Limited
1993 Mazda Miata
2005 Basset Hound, "Fred"