This is the method recommended by ARB. For really bad mildew it will require removing the tent from the platform.

Mildew can develop any time your tent is stored wet. It looks bad, smells bad and can damage your tent’s waterproof coatings. DON’T LET IT START.

Alas, if it already exists, here’s how to approach it:

• Try some light scrubbing with a sponge during a regular cleaning session.
• If mildew is still apparent, mix 1 oz. of MiraZyme® (or similar product) to 20 gallons of water in a bathtub and dip the whole tent.(Note: Lysol® also works, but its scent is attractive to bugs and critters and is thus not recommended).
• For spot treatment, use 0.5 oz. of MiraZyme® (or similar product) per 1 gallon of water and thoroughly scrub afflicted areas by sponge.
• Set up the tent in a shaded spot and allow it to air dry.
• Then, mix 1 cup of salt and 1 cup of concentrated lemon juice with 1 gallon of hot water.
• Rub the solution into the visible mildew and once again, allow the tent to dry.

This procedure will stop mildew growth and eliminate the odor, but it will never remove the stain.