Where are the wastewater holding tanks located at in relation to the CG? If you are transfering weight (pee and blue juice) into a holding tank that is aft of the aircraft CG, then that should reduce the amount of tail-down force required to offset the natural nose-down moment created by the CG being forward of the center of pressure on the airfoil.

In basic aerodynamics, when in straight and level unaccelerated flight isn't total lift assumed to = total weight? Total lift must support not only the weight of the aircraft and all its contents which is "focused" at the CG, but also overcome the balancing effects of the tail down force.

If people going to the lavatory to pee results in the CG moving aft, and less tail down force is required, has that not reduced the "total" weight of the aircraft?