We have a 1987 3207. I've poured sewage treatment liquid into both tanks after every pump out since we've had it. Lately, there is a god awful sewage smell every time either unit is flushed. It comes from the stern and is even noticeable while under way. The tanks must be vented out the stern but I'm not sure. I've seen in-line filters on various sites and wonder if they work? Does anyone with a 3207 know how the tanks are vented? If there are vent lines, where do they run and what size are they? I've looked through the manual but the information is very generic. Any help is appreciated.