Don't discount your gray tank as the possible culprit for odors. We religiously flushed/ treated our black tank, but after about 3 years on our new rig, we started smelling the "ew!". We now treat both tanks with the Dawn/Calgon mixture and installed these new vent caps, which has solved the issue.
360 ventOh, and Skynyrd is the best! Saw them in 1977 sandwiched in between Black Sabbath and Rod Stewart ( with the Tower of Power brass section), awesome show. Skynyrd playing Freebird for 30 minutes stole the show! And yes, my ears are still ringing......