Are you sure it sewer smell? If you haven't put any chemical in your kitchen gray tank in a while you could be smelling decomposing food particles. This can happen over time if you regularly leave your gray tank open when on full hookups. Small particles build up in the tank and when you then close this tank when moving or boondocking these particles hydrate again and start stinking. Believe me it smells as bad or worse than a smelly black tank.
Been there and done that many years ago. Now I just keep my gray tanks closed all the time just like the black except when they are full and dumping them.