Where is it being stored? Southern States, Northern States? What's the weather conditions been like where it's parked? If you're in a humid area, leaving the vents open will let humidity in as well as let it out. Hot and cold changes will cause condensation to form, and if there's no air circulating, it will continue to evaporate and re-condense. If you are in cold weather, it will turn to ice crystals inside the camper.
So, without knowing where you are storing your camper, and under what weather conditions, it's kind of hard to give a cure. If you can get some heat inside, it might dry it out. But if you are in cold weather, as soon as the hot reaches the cold, you'll have condensation again.
If you're in freezing weather, a dehumidifier will probably be of little help, unless you can can get it dried out and then get it closes up.
So, depending on where it's located will help answer with a possible cure.