While you're at it, give the holding tank a thorough rinsing and deodorizing. You are not alone. Chances are that if your boat was buttoned up tight for the winter with little ventilation, mildew is alive and well below. Use a mild to medium solution of chlorine bleach and water to remove the existing mildew and help rid the boat of mildew's associated musty odor. If you can't keep portlights cracked, install Dorade vents, small saucer vents, or solar vents in deck or hatches.
Mildew loves moist, still air.
If AC power is available, purchase any of the electric dryers that are available and put one at each end of the boat. The whole purpose of the dryers is to heat and dry the moist air and cause it to rise and circulate the rest of the air inside the boat. Even something as simple as a 60 watt lamp, as low as possible, will help. The heat tape trick works really well under mattresses and between the mattress and the hull in the v-berth area. It's time to give some attention to your fresh water system also.