Not sure what make of slide topper you have (Dometic or Carefree) but you might want to visit the manufacturer's webpage to see if you can find any information there. Failing that, try doing a Google search for other resources that may help. Google "tightening RV awnings" or similar. Some other members might also chime in with suggestions for you.
Over time, the spring mechanism may need adjusting and; as well, the fabric itself will stretch somewhat resulting in a sagging topper. A couple of members have purchased some type of support that they insert between the slide roof and the slide topper which provides a lift to the fabric. The support that was used is called an Air Wedge ...check out their webpage ......
www.awningairwedge.com
Hope this helps
( I was told that it never rains in Sherwood Park........???)