I live in bone dry Calgary, Canada. Only rarely do we get mildew.
In humid eastern North America, however, my tent starts stinking the very day I put it away (slightly) wet.
Here I am trying to dry it out over an indoor heater:
Any tips on how to prevent mildew?
besthike.com 
I have mildew on my tent after forgetting to dry it. My backpacking buddies always joke saying I’m going to get black lung sleeping in my mildew tent!
Usually after each trip, I let my tent “air out” outside and take a damped sponge, wipe both inside and out. After it’s completely dry, I store it in a cool place.