My guess is there is a hole somewhere on the black tank. Maybe it is high enough that it wont create problems though. I did the same thing with my last trailer. Lucky for me, the tank is exposed underneath. In my case I went into the house and forgot I was filling it. I heard a loud bang and found a steel strap had broke off that supports the tank. The tank expanded enough to blow that off. I also had some water come through the floor inside around the toilet base. It never caused any problems though after that.