Mildew is a real problem and prevention is the only real way to deal with it. A tent must be packed away bone dry with any potential sources of damp removed. This could even be from insects that have been packed away with the tent! Or from the core of guylines that were not dried unravelled to ensure deep moisture was removed by air flow.
Once mildew has set in you have to stop it spreading. Kill off the spores with a very mild solution of one part Milton or white vinegar to ten parts water − rinsing off well. Marks are harder to remove and we have not yet found a satisfactory solution although we have read a few ideas on Facebook from Outwell fans. Whatever some can be quite aggressive so be very careful...
Besides the cleaners from the likes of Storm you can get canvas cleaners from the caravan market and mildew cleaners for sails from the yachting market but we have not tried any of these. The Camping and Caravanning Club used to have a data sheet on tent maintenance - worth looking on their website.
Remember to reproof the tent after cleaning.