Ventilation is the way to go. It seems self defeating to open a window when running the heater, but it works. Cooking is productive of far more moisture, since the heater flame is isolated from the cabin area, but the stove is not. turn on the vent fan the second you turn on the stove to cook. Secondly, human bodies produce a lot of moisture from both the skin and from breathing. Two adults and a couple of grandkids can steam up the windows pretty fast. Imagine what that is doing to the bedding, and clothes inside the closets which are trapping moisture against the walls and shelves.
In Elk Camp, where it is warm and dry during the day and cold and dry at night, we battle with that every day. The second the sun strikes the RV, we open several windows and air out the dampness. Work with it, and be sure to check inside closed area like closets and cabinets.