With the original fiberglass gel coat, removing the mildew and other staining was a major operation each spring. Seemed that the warming weather in Feb & Mar caused the stuff to really grow. I tried various soaps and even pressure washing. I finally settled on ordinary bleach cut about 50% in warm water spread on with an auto wash brush on a long pole. Best time is on a cloudy or drizzly day. Once the mildew turns white, rinse it off. I use an auto wash brush connected to a hose.
A couple of years I did cover it with a blue poly tarp. The problem is I use the trailer more often in the winter months than other times of the year. Taking the tarp off without dragging it on the ground and getting it all dirty with grit is impossible, even with a helper. It seemed that there was more gel coat damage using the tarp. Perhaps one of those soft tarps wold help.
Last year I painted the Scamp
with auto paint
and clear coat and this spring, NO MILDEW. The stuff is shiny enough that there isn't any pores for the mildew to get started in. I don't know how long this will last, but I have a car over 10 years old with the same paint
system and it stays free of mildew.
As others do, I keep the top vent open slightly and all the windows
the same. There is also a computer fan that runs constantly. The elephant hide surface did collect mildew some until I painted it with a mildew resistant paint