It depends, Stu...
If you have no choice but to hold toilet waste AND do not have immediate access to open ocean beyond the 3 mile limit to dump the tank, it makes sense to have no y-valve anywhere, 'cuz pumpout is the the only legal way the tank can be emptied. That would be the case in LIS, the Chesapeake Bay, SF Bay, Puget Sound, all the San Juan Islands except for the Strait, Galveston Bay, the Great Lakes and all inland lakes and rivers.
If you do have immediate access to open ocean outside the 3 mile limit, where to put the y-valve depends on whether you spend most of your time in close in coastal waters and only go outside the 3 mile limit to dump the tank, or whether you'e a "blue water" cruiser. If you're a coastal sailor, put it in the tank discharge line--to provide a choice of pumping out or dumping out. If you're a blue water cruiser, put it in the TOILET discharge line to provide a choice of flushing into the tank or directly overboard at sea.
Shell, you just answered your own question:
I don't understand why but a rebuild kits for many of them cost almost as much as a new toilet. Sometimes replacing the toilet is a lot less work than rebuilding one.
It's because there's more profit in toilets and toilet pumps than in repair parts. Cheap toilets also don't last very long, so they sell more of 'em.
And btw...you'd find it a lot easier to maintain your tank and also to prevent odor if you replace the too-small vent line with a 1" line (not hard to do ) and get it out the stanchion and into a nice open thru-hull that'll allow some air exchange and allow you to backflush it every time you wash the boat to prevent it from ever becoming blocked. Catalina is the only builder who vents waste tanks into stanchions...and even they've finally recognized that, while that may be ok for water and fuel, it's a horrible idea for holding tanks and started venting out thru-hulls instead.
