Maybe the auxiliary tanks are for the crappers!
That's definitely on the cards, some new design features in future aircraft will be the ability to make water in Flt, reducing the necessity to take off with large quantities of water, reducing takeoff weight. And you are close to the mark with sharing aux tanks for sh!t. There is talk of having a bladder in the tanks that allow waste to be stored as fuel is used. Dumping waste has also been discussed, so far they are only considering the excess fluids and not the solids.
Blue rain over Bahrain