I'd have to ask, how much ventilation does your boat have? The biggest cause of condensation and mold/mildew is usually insufficient ventilation. Adding a few solar fans or dorades may greatly improve the situation without impacting the energy budget.
SD, the ventilation is not as good as I would like, but I have added some efficient internal fans to keep the circulation going. The design of the boat is not great for dorades and I was concerned about installing a solar fan after I heard that lines can easily catch them and pop them out. My lines are running where one would normally install the exterior ventilation, so I am pretty much limited to my hatches for ventilation.
I do not have the problem during the summer, just the winter when I am running the heating, and since I run the heating at the dock, the dehumidifier is not too bad of a solution right now.
Where does the rain go that gets down the mast. That seems to be the most likely way for moisture to get in a boat. Get rid of that water and you'll have a dry boat. Ditto for any deck leaks, which are a problem in themselves.
My mast does not come through the boat; it stops at the top of the cabin (benefits of a rotating mast), so I am not getting moisture from outside, just condensation of the inside air in the evenings when the heater is on, maintaining a higher humidity inside than outside. The fans keep the inside air more uniform and prevents condensation on the cooling hull, and the dehumidifier seems to be doing its job by maintaining a low humidity inside. Actually, I found that I now have no significant condensation on the metal edges of my hatches using this configuration.