It looks like you're talking about a slideout cover? The way you pose your question, it's possible to think you're looking for a traditional awning for that 20' slide. If it's a slideout cover, then Carefree of Colorado has an awning configurator program online. You can build the slideout covers using the configurator then search using google by entnering the part numbers.
All of Carefree's products are linked at this page:
http://www.carefreeofcolorado.com/rv...l-trailer.html
Carefree makes 4 levels of slideout covers. We have the "cheapest" ones. They are not cheap and they've been on the Cougar for almost 5 years with no problems. The only covers available in "extra length models" are the Alpine or the Ascent. Both of those are available up to about 28'. The Slideout Cover and Slideout Kover 3 models only range to about 18' long.
You will probably need the TALL mounts to position your cover above the flanges. I think Keystone uses wide slide flanges on all their products. Any question, just ask and good luck !!!