Ventilation and heat. We keep a space heater set to 50 degrees in the truck camper, no problems. And we keep the furnace set to 50 degrees in our park model. The furnace air exchanger works for ventilation. In our travel trailer we kept a space heater set to 50 degrees and a couple of windows open. Those are storage options. When you are there you, by breathing and cooking are pumping more moisture into the air than when it's empty. You'll have to allow for that. Maintaining a consistent temperature will help some. We are near Lake Stevens and humidity has never been a problem with those things in place, never needed a dehumidifier. This has been a warm, wet winter, not a good combination and that could be part of your problem.