I took a different approach based on advice from Gary and Renee--add bleach every time before I dump the grey water tank. I add about 1/2 cup bleach mixed with about 3/4 cup water to the grey water system -- spread around between the sinks, shower, and bathroom sink drain, and flush it with lots of fresh water. I usually do this before I drive the coach to the dump, or if sitting still, for a few hours before I dump so it gets sloshed around.
I used to have the H2S/hydrogen sulfide smell problem before I started adding the bleach. Haven't had a problem since. I use formaldehyhde-free holding tank deodorant in the black water tank so I haven't had a problem with it, but Gary tells me he adds bleach to the black tank, also, instead of holding tank deodorant and it works fine.
So if you aren't ready to add the swivel vents, which others have also told me works well, try a little bleach in your tanks and see if it works for you like it does for us. Just remember to mix it with water and flush it with a little fresh water because bleach can be corrosive to stainless steel.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra