I havent read all the responses, so please forgive me if this post is redundant.
Your problem is what folks round here call a "boat fart" which is characterized by a distinctive acrid stench. I have had problems with them getting into the cabin three times and all three involved the vent.
The first time, a dirt-dauber had blocked the vent at the hull fitting. The second time, I had overfilled the tank allowing some of the muck to get into the vent fitting at the tank. When the muck dried, it blocked the vent. The third time, I had replaced the old marine air unit (located in the bilge area) with a larger one during the summer and suddenly the boat fart would get in the cabin when the toilet was flushed while the a/c was running. It turned out that the return air vent between the cabin and the bilge was too small, forcing the a/c unit to also pull in outside air through the engine room vents in the hull. One of these vents just happened to be right next to the holding tank vent, so the boat fart was getting sucked into the boat and circulated by the a/c fan. A larger return air vent in the cabin floor solved the problem.
Your problem could also be caused by switching from a manual toilet to an electric one because the electric forces a larger volume through the hoses at a faster rate, thus overwhelming your tank vent.
If you can smell a very foul odor outside the boat when someone flushes, your problem is probably not the vent. Another test is to observe the holding tank while someone flushes - if the tank appears to "balloon" outward, your vent is blocked. Yet another symptom of a blocked vent is the presence of foul-smelling bubbles in the toilet water for a minute or so after each flush.
Many boaters find that trying to suppress/eliminate tank odors can be an expensive and often futile endeavor. Putting up with boat farts - which usually dont last long - is IMHO just part of life afloat.
Finally: If the odor you are experiencing is more of a rank smell than the one described above, your problem is more than likely due to "permeable" waste system hoses. If your waste system uses flexible hoses between the toilet(s) and the waste tank, you will eventually have a permeation problem. You can make the hoses last longer by always flushing enough water through the system to ensure that no waste remains in the hose. However, this tends to fill the tank much more rapidly. One great solution to the hose problem is - if possible in your boat - to replace the flexible hose with rigid PVC which is impermeable. The other is to use a marine sanitation hose like that made by Vetus. The downside of this is that the product is pretty expensive ($7+ per foot) and doesn't last forever. If you do choose to replace with hose, wrap it with aluminum tape, if possible, because aluminum is also impermeable.
Hope this helps