USCG reqs require that all waste tanks
be vented to the outside of the boat
. So if yours vents into the anchor locker
, it's vented illegally.
The "standard" vent line size is 5/8", but that's only because it's always been the "standard" size vent for water
and fuel tanks
so the industry just assumed it's also the right size for waste tanks. They're wrong.
You're on the right track with a 1" vent line, but as long as it's short--<5'--straight, doesn't rise more than 45 degrees, and the thru-hull is an open "bulkhead" or "mushroom" thru-hull, one is usually enough. Even better if you can run it forward to a thru-hull about a foot below the toe rail. That would keep air moving in and out of the tank any time you're underway, or on an anchor
or a mooring
All this is described in detail in my book, btw (see link in my signature, just click on the title) and I'm always glad to help you sort out the best way to do it.