We have a single window awning and prefer it to the combination topper/awning models on our slide-outs. The window awnings seem to be a lot sturdier than the giant "shade sails" and are just right for keeping the windows open during light rainstorms. One issue with retrofitting, though, is having solid wall structure where you want to mount them. If the RV manufacturer didn't offer window awnings, it seems unlikely that they would take the time to design and build the supporting structure into the exterior wall.
2000 HR Endeavor
2000 Holiday Rambler Endeavor 40PBD
Freightliner XC, CAT 3126B