I don't think the vent being on the same stack as the drains is much of an issue. I see the same in houses all the time.
I'd start with flushing the grey tank really well. They do build up debris after all (hair, bits of food, etc..) Fill it all the way and then dump a few times the next time you're at a full hookup site or at a dump station with some time to kill. Then add some deodorizer, either the GEO method or some basic TST or whatever you happened to use for the black normally is probably fine. Drive that around until your next camping trip and then see if the grey still smells.
There's no use crying over spilled milk, unless it's on your keyboard.