I use a product from IOSSO called Mold and Mildew stain remover that you can find online or here in town at Seattle Fabrics.

Mix the powder with water and spray it on the underside, rub it in lightly with a sponge and let it sit for a bit.

Rinse off with clear water.�

Never done this with the top on, only with the top off, upside down on saw horses when changing out the tent.

May try the Zep mold preventative stuff, but the IOSSO bleaches out the moldy bits quite nicely.

http://tinyurl.com/cgb5wp9

My van is parked outside, on the street, but other than some green slime on the outside, doesn't get too much moisture build up on the inside.

Cheers,
Marco in Seattle