Mildew on canvas Annexe walls

We have mildew on the annexe walls of our camper trailer. The main tent is no problem. What is the best way to treat it ? They have been dried well and truly in the sunlight but there are still lots of yukky suspicious lurking white spots. A good scrub with plain water looked good but may compromise any water proofing. Of course we can re proof I suppose. We cannot store it under cover so perhaps that is also a consideration for type of treatment. Need advise on proofing and anything else I can't think of.