Found: 1919 Newspaper Ad
Popular periodicals of the 1900′s almost always contained car advertisements from the major car manufacturers. The advertisement below, distributed by the Oldsmobile Sales Company in the June 29, 1919 edition of the Washington Post is probably the very first advertisement in which the Rockville Garage appeared. (click image to enlarge) Initially, Reed Brothers sold Oldsmobile and Hudson, along with Dodge.
Dealers are listed in fine print at the bottom: note Rockville Garage, Rockville, Md, which I have highlighted in yellow.