I mounted my 16 gallon grey tank is just behind the axle
between the frame rails.The rear floor of the Casita's steps up about 4" in the rear dinete area which allowed me to tuck the tank up pretty high. In addition to lifing the trailer 4 1/2 inches, I also added 1 1/2 inch thick spacers between the body and the frame rails to add more room for the tank. The galley sink gravity feeds to the tank. The shower drain at the front will require a grey water pump to reach the tank.
Here is a pic of my tank installation. Note the drain plumbing from the sink on the right hand side.