I have a similar problem I just noticed this weekend on an overnight fishing trip. Boat is a 06 Marlin with fresh water toilet (and FW holdng tank). First let me mention the water (hot or cold) has virtually no smell or foul taste coming from any of the four faucets. There is no smell or ring left in the toilet bowl from the fresh water. I think if I had to, I could drink the freshwater and I would be fine (although I still prefer bottled water on a boat).
Before leaving the dock, I filled the fresh water tank. A few hours later on our way to the canyons we emptied the holding tank. About an hour later I remembered to add the powdered toilet stuff and flushed the toilet to as per the instructions to deoderize the holding tank. When I came out of the bathroom the crew was complaining about the awful stink (there was no odor in the cabin or bathroom area). A few hours later someone used the bathroom again, and I got a whiff of the odor they were talking about - it really was bad. I assumed the vent on the holding tank is allowing some 'sewer' gases to escape, despite the fact that the holding tank treatment was just added. I would have expected it to smell like pine trees since the fluid in the holding tank was highly concentrated with the chemical, not rotten eggs? Did not notice the smell again for the rest of the trip or back at the dock. Any ideas?
Dan, maybe you have the same boat as me and we have a similar problem? I did not see what boat you have, but if you have a saltwater flushing toilet as suggested, I guess we do not have the same problem.