Clove oil is very popular down here for mold/mildew defense in closed up/tarped up trailerboats. Kills the spores and all. 1/4teaspoon to a litre of water.Spray on, let sit for a minute then wipe off. Smells a bit, but that will dissipate.
Never had a mildew problem in a boat myself, but a few guys I know have - and a lot more people on the various fishing/trilerboat forums down under swear by it as well.
Heres an ABC article with 'mould expert' Dr. Windsor
- funnily enough he says to be cautious of the clove oil due to the potential for staining. I've never heard any of the supporters of clove oil (many many of them whose posts I have read) mention staining.