Hello LMH,
Not withstanding mcghee's comment and implication that shut caravan windows are not a significant improvement in security over a widow set on ventilator, I venture to suggest that your insurance company would take a different view.
Provided the ventilation grills are kept clear so that air can freely flow in and out, then there should be no need to leave windows partially open.
If you do find that damp is a problem, then it could suggest that you have water ingress somewhere.