I know little about sea going boats however with two toilets I would have two tanks and train the crew to use one toilet first untill the tank level gauge of which there are various types shows it to be full then start using the second one until the first can be emptied.
Stainless costs more but are better and last longer than plastic tanks BTW. Also have inspection/ access hatches fitted to top of the tanks. Re refilling, mine doesn`t have a cistern as such but does fill the toilet bowl with water after use.
On all waters be it canal, rivers or sea the grey water from sinks. basins and showers is allowed to exit through the side of the hull via plastic or brass skin fittings, brass being stronger and to me looking better, just site the fitting a couple of inches below the basin exit point and connect with plastic pipe ( don`t know why but my kitchen sink has a trap with the basins being straight through). The shower has a gulper pump with the exit skin fitting at the same height as the other sink/basin fittings with the on/off switch inside the shower room (yes seperate shower room being all posh
but also means nobody is banging on the door asking when you are coming out!).