I was wondering the same thing, but looking on the crew list, every Saloon Steward was said to ha e been situated in First and Second Class. Perhaps the Steerage stewards (and possibly Matron/Stewardess Wallis) helped served the food in the dining saloons and did other stuff in helping the steerage passengers (the Kosher cook onboard did more than just cook food for the Jewish passengers, so why not with the stewards).
Like I said, I'm not exactly sure, but I'm sure that Bob, Ioannis, Adam, and the others on ET have more on the stewards (and possibly the matron stewardess)