Okay- there is something worse then finding a tear in the screen of your pop-up trailer! Discovering mold or mildew growing when you open it up from winter storage.
Mold or mildew could have started because of a leak- but most likely cause is storing the camping while the canvass was still damp or not totally dry.

You can get rid of mold and mildew stains from your pop up trailer- but you need to be careful you don;t remove the water proof coating. Call your dealer or manufacturer to see if they have any suggestion for removing the mildew stains.

The best products for camper canvass are are bleach ALTERNATIVES- not bleach. Bleach will ruin the coating and may also damage the fabric.
To start- first get rid of existing LIVE mold and mildew. Put your camper out it direct sunlight. You can then “brush off” the mold and vacuum up ALL the spores ( and I would throw out the vacuum cleaner bag when you are done)

Solution for Removing Mildew Stains

Use a solution of 1 part water and 1 part rubbing alcohol and rub over the stains. This should remove the stains for both mold and mildew effectively.
Make sure you test on spot where if it changes colour it won’t show.

You can also try making a paste of lemon juice and salt and let sit in the sun- this works very well for getting rid of mildew stains on fabric without using chlorine bleach.