Take a look at my simple set up:

http://home.maine.rr.com/rlma/Strider0601.htm#Tee

(and continued on the following page)

No valves other than the discharge seacock. I used a 1" Bosworth pump
and it works great. The O-ring in the deck pump out is tight enough
for the pump to draw up into the Tee in the deck pump out line and I
haven't felt anything that makes me think the discharge needs to be
larger.

I didn't use a vented loop because the discharge seacock is normally
closed. Even if it was left over, back flow would have to go through
two pump oneway valves, fill the tank, backup into the head discharge,
overwhelm the joker valve, and then come up through the toilet. It
could happen over time but it's not a big enough risk to hand pump the
extra distance and possible odor through the siphon vent.

I removed the seacock handle and cable tied it to the discharge hose.
This is sufficient to show the USCG intent not use it inshore or
permit accidental discharge. I turn the seacock with vice grips when
I do use it three miles out.

--

Roger Long