How big does the waste tank vent hole need to be?

Rvpapa wrote:
Glass in a threaded hub on the tank. Cut off a section of vent stack in closet. Install male threaded end on cut-off. Trim to appropriate length. Screw into tank using sealer on threads. Join to stack with M-J clamp.
Art

That's why I come to the forum. Sounds perfect, thanks.

threaded pvc fitting, fiberglassed in. Little messy, but does the job.
This is actually better than original since it can be removed without cutting or breaking if tank needs to come back out.