This is a reprint of another article on the topic
Before using any product to remove mold and mildew stains from the canvas of a popup camper, start by drying the canvas in direct sunlight. The sunlight will dry the canvas, and it will kill any live mold and mildew spores.
After the popup camper canvas is completely dry, brush the mold and mildew-covered canvas with a wisk broom or other stiff brush. Vacuum the canvas to remove the dried mold and mildew, and continue with a cleaning method suggested by the manufacturer of your popup camper, or you can try the following homemade canvas cleaner.
After brushing and vacuuming the canvas of your popup camper, mix a solution of half rubbing alcohol and half water. Test the solution on an inconspicuous area of the mold and mildew-stained canvas. Simply apply the rubbing alcohol solution with a white cotton rag, and if the solution does a good job and doesn’t compromise the canvas in any way, continue cleaning the rest of the mold and mildew-stained canvas.