RV condensation in the winter is a pretty common problem. Cooking, showing and washing dishes can add moisture into the small area of an RV. You can try different methods to see if one will work for you. Crack open vents when cooking and showering. If you do not have double pane windows you can use shrink wrap plastic on the inside to help keep out cold air, which will contribute to the forming of condensation. Another more radical option is to cut a piece of insulation and place it inside of the vent and then you can cover with shrink wrap plastic to seal it off. The use of electric heaters such as the fireplace and a standalone unit can help reduce moisture. Lastly you could always run a small dehumidifier. Hopefully your moisture leak is not coming from the outside around the vent. You may want to make an inspection on the roof.
Dean & Pam - SOB