Missing? Y valve
By gosh, you're right! My 2000' manual shows a Y valve on the plumbing diagram, with one hose leading to the holding tank, through the macerator, and out the seacock. The other hose goes straight to it's own seacock. I've never seen the second hose or seacock.It's interesting that the manual's written text instructions do not refer to the Y valve either. Step three under the title "THE HEAD" says, "Waste will be pumped directly into the holding tank when the bowl is emptied." But I guess I'll have to take another look when I'm there next time anyway.