There is also a PVC check value in the galley sink plumbing that I believe is designed to let water down and prevent odors from coming back up to the sink. Sounds like that $5.00 check valve may need replaced. On my new RV, the manufacturer dropped some glue on the valve preventing proper operation. This was in my bathroom sink and we could not find the foul odor under my mobile mechanic showed up and stepping out his truck said he knew exactly what it was.
Bob & Sue Rowan
2012 Coachmen Mirada 29DS (SOLD)
2015 Keystone Bullet Travel Trailer
2016 F-150 Lariat with Auto Trailer Backup
Dog = Morki (Yorkie-Maltese) - Zoe