Guidelines
Hose from the toilet to the tank shouldn't be much more than about 6' long. V-berth is the best tank location unless it means the hose will be too long.Spec the inlet (hose from the head) and vent fittings toward the centerline of the boat, away from the hull to prevent tank contents from flowing back toward the head and spills out the vent when heeled. If possible (1.5" elbow needs at least 5" clearance above the tank, 1" vent needs at least 3"), spec all the fittings on the the top of the tank, with a pick-up tube inside the tank on the pumpout fitting. Go to the link below (so much of the other info in it is out of date that my publisher let me leave that one up), scroll down to the Odor Control section for more details.Search the archives here for previous discussions.