When we purchased our 34C Silverton, the forward cabin walls, which are fabric, were all mildewed. I used our "green machine" carpet cleaner and applied stain remover....no luck. I tried using some tilex (in a spot where it wouldn't be seen) and scrubbed, then used the carpet cleaner and VIOLA! The mildew came out and didn't leave a white spot. I did the entire cabin and the difference was amazing. The fumes were miserable even with the hatch open and the fans blowing.
Alot of boat manufacturers use olefin fabric and carpet and that can be cleaned with bleach-based products. Be careful, be sure to test in an unseen spot and make sure that you go over the areas you are cleaning a few times so that you extract as much cleaner as possible. Alot of stains are caused by moisture left from unextracted moisture.