Speaking of vents::

If your going to combine any vent lines, be sure you they won't create a
syphon between port and stbd tanks while healed over.

What I did to over come this problem, was to locate each vent fitting at the
outboard edge of the tank top. That way the higher tank will always have the
vent fitting in the resulting air pocket while, even though the low tank
vent might have water up to it, the opposite tank will still be higher due
to the boats heal angle. This wouldn't work if the vents were vented at or
near the centerline.

All of my tanks are at a equal tank top height, fore and aft, port and stbd
while the boat is on an even keel.

I run my combine water vent to the shower stall.

Another feature, I'm working on. The water heater is in the engine
compartment, about 20 ft from the head and shower. I am putting a valve in
the hot water line, near the head sink, that will divert this water back to
the freshwater manifold until hot water is up to the fixture. This may seem
'anal' but it will save a lot of wasted water while waiting for the warm
stuff.

However, speaking of combine vent:: I did end up with a problem of cross
over between my holding tank and my gray water tank. They share the same
vent and even though they are tee'd to gather about 3 ft above the tanks,
when ever the holding tank is full (I don't have gage) the liquid flows over
into the gray water tank. Eventually I will discover the problem when the
gray water backs up into the ice box drain. YuK.

Well, that's my take and experience on the tank plumbing situation.
Live(aboard) and learn from your mistakes. FWIW.

Steve
s/v Good Intentions