Correct about both being under the bunk. On ours, AC right under our pillows, then PotH20, then black water tank as you move from port outboard inward to midship. You can see the black water tank from 'inside' the transom locker area ... follow the hose from the pump out connection or the hose from the aft head vac-u-pump to the aft side of the tank.
Changes in L'Attitudes
1999 Maxum 4100 SCA