Options....
You prob'ly don't have a flush water intake thru-hull either, so unless you want to install one, your options are: 1) an electric toilet designed to use pressurized flush water plus a holding tank and all related plumbing;2) a permanently installed 5-6 gal portapotty fitted for pumpout...not a bad choice at all, 'cuz it's self-contained--so no plumbing except a vent and pumpout line--and because it uses so little flush water, holds about as many flushes as a 12-15 gal tank connected to most marine toilets.3) if there's room in the head for the tank footprint (approximately 20' x 20"), the SeaLand 711-928 Marine Traveler--which would be my first choice...also self-contained. Has a real china toilet mounted directly onto a 9 gal tank...uses pressurized flush water--so no sea water odors--and so little of it that it holds about as many flushes as a 15-18 gal tank connected to most marine toilets.