It will be helpful for you to know that Titanic's crew (and other ships) was separated into three departments. The Deck Department, along with the captain and officers (who were sometimes counted with the Deck Department, for a total of 66), was responsible for all the nautical tasks, the work that is specific to ships. It included the quartermasters (who, among other things, operated the helm), lookouts and seamen. They also had duties related to launching the lifeboats. The Victualling Department (it's pronounced "vittleing") served the passengers; it included the cooks, stewards, waiters, and so on. The Engineering Department was responsible for the boilers and the engines.
The living accommodations were very much separated by department, by job, and by status relative to other crew. The Deck Department had quarters on E Deck, close to the bow. The Engineering Department had quarters in the bow on D, E, F and G Decks, although the engineers themselves had their own cabins on F Deck near the reciprocating engines. The Victualling Department mostly had cabins along "Scotland Road", the long passageway on E Deck, along the port side of the ship.
If you look at the crew's quarters on E Deck, you'll see that the cabins were organized by job title. In general, the higher-ranking crew cabins had fewer occupants, and a few even had their own cabins.
Your speculation about restaurant staff as the highest-ranking crew is, sadly, not correct. The restaurant staff were employed by the operator of the restaurant, Luigi Gatti, and not by the White Star Line. As a result, they were neither passengers nor crew. The musicians were in a similar predicament, and the two groups met a similar fate, although they do not have a similar legacy.
Only three out of the 69 restaurant staff survived: two female cashiers and a male kitchen clerk. As you know, none of the musicians survived, but they will be remembered forever as the band that played on. The restaurant staff perished in a way that was as ignominious as the loss of the musicians was glorious.
Several accounts claim that the restaurant staff were locked in their quarters on E Deck. The survivors were evidently wearing ordinary clothing instead of their restaurant uniforms, and were considered passengers by the WSL crew.
