Fresh water has vent,fill and out holes.
The black looks like:big hole on top toilet,big vent on top(2in?)....big hole on end(bottom) is drain...3holes on end mabye level sending units.
As long as they are tapped any of the small holes can be "plugged"
Also extra small holes in black tank opposite drain could be spray down vents(to wash out goop)
__________________ Our build La Tortuga Accept the challenges so that you can feel the exhilaration of victory. George S. Patton