Hoping this thread is still semi-active.
I too hate the idea of flushing what was once a second ago fresh water down the drain. For us, historically, fresh water was the limiting reagent on how long we could boondock....in that vain we are fans of composting toilets and have experienced them on treks in NZ and believe it or not here in the U.S. on the Appalachian Trail.
So in a
Casita (which we are strongly considering ordering), if you did forego the use of the black tank as a black tank and wanted to have more freshwater how hard do you think/guess/estimate it would be to place an auxiliary fresh water tank in its stead? I would think it fairly easy to fashin a 12vdc pump to move water from one to the other and that might even be optimal for balancing the tongue
weight 
I think submarines shift water for balast control.