Answer:
The most inexpensive way is a good old-fashioned strong mix of white vinegar and water. In the tub, soak the tent in just enough cold water to completely cover. Then add about 1 gallon of white vinegar. (Note: You might want to open a window or two, if possible, if you choose this method.) Rinse and dry thoroughly before repacking or better, let tent hang in a place with plent of airflow until you're ready to use it again. (Mildew requires limited oxygen to thrive.)
I've also had good luck with Oxi Clean. Make a paste according to manufacturer's directions, apply to affected areas, then scrub with an old kitchen sponge. Rinse thoroughly and let completely dry before repacking. This method is easier if you can spread out in the backyard and then rinse with a water hose, plus it keeps the mildew out of the house. The tub will work, though, if you're in a pinch.
Both methods should kill the mildew, but vinegar may or may not take out any stains. Oxi Clean will likely take out most of the stains and doesn't leave a funky smell on your tent.