Venting is neccessary to enable fuids to move into and out of tanks.
Vents also reduce odour, if efficient enough, allowing oxygen into the
mix, preventing anerobic reactions, which actually cause most of the
stink.

If you vent the pumpout hose, how will you maintain "suction" when
pumping out? A vent valve might work, but needs management, is all.
Imagine being at the fuel dock, ready to go except the pumpout isn't
working. One distraction and you could hold up the works because the
pumpout suction line vent is mysteriously open. Besides, the vent
won't breathe the tank if it is in the suction pump out line.

I don'tsee any benefit in the idea.

The tank vents are better connected directly to the top of the tank,
and should be roddable, to clear them as required.

I made my own perfectly fitted waste tank from leftover glass and poly
resin, using a stitched together, waxed wooden form that came apart
inside the finished tank, before forming a flange from bog to which was
sealed the top of the tank. I use a compressed air system to blow the
tank when dumping at sea, or into an external toilet hose connected to
a floating barrel for near shore self help pump outs, or to Wall Mart
parking lot storm drain or service plaza, as is so popular with the RV
crowd.

The only in line sewage valve is a plasic ball valve at the marine
discharge through hull. When closed ("port" position) toilet goes to
tank. When discharge valve is in the "sea" position, toilet waste goes
to the tank and sea until the tank level is even with the waterline,
unless to vents are closed, in which case, the system "goes" to the sea
once there is any overpressure pressure in the holding tank. So, I can
manage ballast as well as is done in a submarine. Usually, the through
hull is closed, as a matter of safety, and is only opened to empty the
holding tank if not using the on deck pump out fitting.

One or two pounds pressure is enough to purge the tank in two minutes
or less, and it is neccessary to have valves in the vent lines to
enable this operation. I use a cheap 12VDC tire pump to pressurize the
system for purging, and an RV air purge shraeder valve in the vent
line, between the vank and vent valve.

When purging, I observe any backwash from tank to toilet bowl, as a
method of monitering the condition of the duck bill, or "Joker" valve.
Any sign of leakage would be reason for maintenance. Opening either
vent valve immediately relieves pressure in the system.

Peggie doesn't like my plan, but I do.

Keep through hulls closed unless in use and monitered!

Terry K