The term "plate steward" are indeed the same as a plate washer.
Trying to match up crew with accommodations shown on the plans - Crew list includes 10 "Plate Stewards" (8 1st, 2 2nd),
One slight correction, first class had seven plate washers in the victualling crew, these being:
First class plate washer Percy Ball (1894-1933)
First class plate washer John Albert Perkin Bradshaw (1869-1912)
First class plate washer Wilfred James Bunnell (1891-1912)
First class plate washer Sydney George Hiscock (1887-1912)
First class plate washer Edgar Hogue (1890-1912)
First class plate washer Frederick William Hopkins (1897-1912)
First class plate washer Charles Edward Light (1888-1912)
While second class had three:
Second class plate washer Arthur Victor Edward Burrage (1891-1951)
Second class plate washers Frederick Humby (1895-1912)
Second class plate washers Arthur Ernest Jones (1874-1912)