Just a thought: was the ceiling vent open? I wonder if a slightly-open vent created a suction in the bathroom as you went down the road?? Might explain how the sink could have black fluid, without having the whole bathroom afloat in it? Or maybe the vent stack is creating pressure in the tank?
I know in our previous Class C, having the skylight vents open on the roof created enough suction, that black-tank odors were drawn into the RV unless we created equal pressure by turning on the dashboard air or opening a window. At least on a Class C, the tanks are farther below the bathroom fixtures (plus we seldom boondocked. The tank was usually empty when we traveled)
