In no particular order I have used in the past...
Meguiars Heavy Duty Vinyl Cleaner
Starbrite Mildew Stain Remover
Followed up with a protectant/restorer.
IF you have it, check your owners manual from Glastron. They may have a concoction of items: amonia, distilled water, and hyrdogen peroxide - that they recommend to clean with. My FourWinns had that in one of the vinyl pamphlets. I don't have it at my fingertips to give the ratios. It worked quite well though!
Edit: Found it on a previous post of mine...
Four Winns had this concoction that I have used with no issues (**This was on my Four Winns so I strongly recommend testing in a small patch first as your mileage may vary with this!!)
(1) teaspoon ammonia
1/4 cup Hydrogen Peroxide
3/4 cup distilled water
Rinse with water and dry....I always used a vinyl conditioner after this as well.