I'm a veteran owner of a popup camper, with canvas walls, which are more prone to mildew than newer fabrics.

As others have mentioned, you have to do what you have to do, which is sometimes pack it wet, but even if it is raining, you can still take a towel and remove as much moisture as possible before you pack it. Then at the earliest opportunity set it up and dry it out. Our camper was up in our driveway many times for that reason.