Got some great advice about venting holding tanks from Dennis Massoth
of Marine Sanitation in Seattle.

"We're now putting in two holding tank vents," he said. "One to port
and one to starboard. No matter which way the wind blows, we'll get
air flow through the top of the tank"

It would be important to install a second vent line fitting, as iar
would just blow through the top of a "T" without really scouring
across the surface of the tank. There's a rubber fitting called a Uni-
Seal that will form a leakproof ring around the new hole cut for the
second vent.

With two vents, there's no "dead end" for wind entering the vent line.

Makes a ton of sense to me. Thought I would pass it on.