Go to an RV store and buy the product they sell for cleaning camper canvases.
How can I remove mildew off my tent without damaging the tent and remove the smell?
Removing Tree Sap from a Tent?
Removing Mildew from Wood?
Cleaning Yellowed Curtains
Vinegar for Removing Mildew from a Tent
Removing Applique Residue from Fabric?
Cleaning a Tent