We had a 28' boat for many years and dealt with this issue. I wholeheartedly recommend an air circulator. It is a plug in unit that is about 10" in diameter with a whole bunch of small holes on the top of this metal thing that is about 4" high and extremely light weight. Just plug it in and we had no mildew or moisture in our boat at all. We now use it in our 21' TT all winter, plugged in on top of the stove and check it weekly and no mildew, moisture at all last winter.
We got it from West Marine a while ago.