I always left a window cracked open and my vent open a crack when in storage. I didn't have a cover over the vent but didn't care since it is a wet bath. Little rain got in and whatever did either evaporated or went down the drain. Some people leave their fan on low all the time if they have power where they store theirs, I didn't. I never had a problem with mold even though we are in a very humid area.
I considered cutting a hole under the bathroom in the pod, and a rear-facing vent underneath. That would have allowed for air flow through the grate just below the bathroom door w/o leaving the windows open when it rained. Never did it.
I did install a fan with a cover on my current camper since it is a dry bath. I plan on leaving it wide open all the time.