Go to any upholstery shop with the dimensions and select the Sunbrella color that you would like to have sewn.
Then, have a mobile RV tech install, unless you think you and a partner are up to the task.
If you are near a boating area, get a marine upholstery shop to fabricate the topper. They will use acrylic thread that will stand up to the environment.
Sunbrella will definitely stand up to the environment, but if sewn with cotton thread it's useful life will be cut short.
Sunbrella is 100% acrylic, will not mildew .
Chose from the marine selection providing you can get a color match.
www.sunbrella.com
My 15 year old coach has all Sunbrella awnings used by Carefree in the manufacture of the awnings , but they used cotton thread which eventually failed.
Dave
1998 American Eagle 40EVS