The moisture is being caused by the difference in humidity outside and the dryness from the heater inside. This difference will cause your windows and probably your walls to condensate. Unfortunately, keeping a vent or window open should help, but it does kind of defeat the purpose of trying to keep the rig warm. To keep some of the cold out you might then want to install covers over the vents like a MaxxAir or other brand. Be careful of this moisture because if unchecked it will cause discoloration and mold.
So a humidifier will work, but most of the ones I've seen work off 110v and you may have difficulty powering it up when boondocking unless you have 110v power for overnight (inverter and recharging system).
We live in the Southwest deserts and camp within those areas, and have been able to avoid this condition, but if we ever get adventurous to stay in humid areas, we'll have to worry about it.
Good luck with this.