I humbly suggest considering a composting system to get rid of all those stinky hoses,
tanks, and valves. But, if that is not what you want to do I suggest
plumbing the
toilet direct to the holding tank, which can then be evacuated be either of two routes: most often through a
deck fitting, and secondly through a pump and
overboard when
offshore. Keep the through
hull handle padlocked closed and you are all set for CG and other inspections. I also think ideally you avoid having a Y valve--instead have two evacuation hoses connected to the tank--one for the
deck pumpout and one for the onboard pumpout. Y valves are problematic.