Star Brite has an excellent mold and mildew remover
I've used lots of products in the past and currently am using the Strabrite product. In many cases it works very well but in some. not so good. I have had good luck with Mold Away too but maybe not significantly better than Starbrite to justify the difference in cost. I use to swear by ZEP mold remover that I would get by the gallon at HD but somewhere along its history, it stopped working as well as it had. I don't know if the formula changed or they watered it down but I don't use it anymore.The mold removers with bleach work faster and better than those without bleach and on hard surfaces are fine but have to be used with caution on soft materials like upholstery and vinyl. They should be thoroughly rinsed off with water since the bleach can yellow some materials ( like gelcoat) or speed the deterioration of the thread on upholstery.
What I have experienced for the last several years is that placing desiccants like DampRid in the cabins really helps reduce mildew but you need to check and replace as needed over the seasons.
On my vinyl cushions, I typically clean them with a mild soap and water mix, dry them and apply some 303 Protectorant. That stuff is expensive buts works very well.
To be fair and upfront, I do not shrink wrap my boats. I use tarps and custom covers that breathe. If I were to use shrink wrap, the proper amount of venting is important. Small solar fans that are cut into the wrap in addition to passive vents can make a world of difference