Black water tank.

[1retired06]

Assume you were filling the black tank using the blank tank flush to get water in the tank? If mine, I would check at the convenience center for leaks, and the fill connection itself which is plastic. The black tank flush valve is also prone to failure. If all that is good, a tank leak would be the next suspect based on overfill. But before I did anything with the underbelly, I would fill the black tank halfway, check for leaks, add water and recheck, until full.