With a few gallons of water in the tank, it can't get cold enough to cause a problem at all unless you put in enough dry ice to freeze the whole thing. How is the fog and CO2 going to get in the rig? And if it could, it would be next to the floor and all you would have to do is open the door(s) and let it out. It would take an incredible amount to fill up the living space and something seriously wrong with your toilet. The gases that form in the black tank would be far more dangerous.