I like your floor plan GE! Its just my opinion but, if you go with one black water holding tank (toilet only), 20 or 30 gal will give you a few days of use untill dumping is necessary.
And one grey water tank (sink and shower), this tank can be of the same size of the black water, or perhaps a little smaller.
Reason being: on the grey water, (depending on where your camping), you can leave the dump valve open and drain the grey water into the woods via a garden hose.
The black water tank will have to be dumped into an approved facility, usually found available at truck stops, rv parks, rest areas, or a home made septic tank in the back yard made from a 55 gal drum.
These holding tanks are usually abs plastic, come in all different configurations and allow for the inlet and vent locations to be placed as needed.