Condensation has been a common problem, discussed elsewhere:
We've had the problem the last 4 weekends we've been camping (its rained every weekend in October around here). Cracking 2 windows and cracking the vent an inch help, but its not perfect. This makes me think more about trying out the small-fan-under-the-shower-mod I've discussed before. The overhead fan is way too big... too much amperage when on battery power and too much air flow. A small 2" biscuit fan, installed under the shower where it would vent up through the vent already installed below the shower would be about right. Only problem would be the possibility of freezing up the water trap for the shower drain if its freezing outside.