If it only happens when you're running some of your fixtures, I'd think it had to do with water being siphoned out of other traps, leaving them open to pass sewer gasses.
Water would only be siphoned if your venting wasn't working properly. I know it is possible to declog your vents, but defer to the pros to tell you how.
To that end, I believe the purpose of the vents is to discharge toxic sewer gasses far enough away from where you can smell them. That's why they're supposed to go through the roof, or far enough away from openings on the side wall. I wonder, therefore, if the vent by the deck should be re-routed in addition. Pros?