For an 88-inch RV slide-out, you should select a Solera 5000 Series slide topper awning that is slightly larger to ensure proper coverage. Adding about 2 to 2.5 inches for overhang, an 8'1" (97 inches) awning is recommended. Regarding the 13.1-foot size, this measurement refers to the frame size designed to fit slide-outs up to approximately 151.5 inches wide. The fabric finish corresponds closely to the frame size to provide full protection. Always measure your slide-out width accurately and order the next standard size up for optimal coverage. You can find these sizes available for order on our product pages.