We've had a stinky galley tank, too. One time the odor was being released by a defective air vent drain valve. Another time the galley tank was so stinky the stink made it into the trailer some other way. We added a treatment called Odorlos that time to the galley and it fixed the problem then.
For the black tank and the galley tank, we now add several gallons of water after dumping and have not had further odor issues. My theory is that the extra water in the tanks sloshes around when driving, and knocks particles off the tank walls, breaks up stuff into smaller pieces, and thus makes the contents ready to be flushed away at the next dump and that this controls the odor.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch