Acceptable humidity level inside TT for winter storage?

Just curious what the ambiant relative humidity is in your area...? We simply leave our roof vents and some windows open year round to get fresh air through the camper. I noticed that our old camper would smell musty if it sat closed up for too long. Leaving front and rear vents open got rid of all the smell. We are treating the new camper to the came treatment, but my outside RH is 34% at 34 degrees right now.