I believe that Breen got the information about Robert Coulton Davis from his obituary, which was published in the American Journal of Numismatics 1888, p. 47. (Available online via Google Books at:

American journal of numismatics, and bulletin of American numismatic and archæological societies, Volumes 22-24

An obituary of Davis titled :Death of Robert Coulton Davis" was also published in the New York Times 26 August 1888, p. 2.

Very brief information at:

431 BIOGRAPHICAL SKETCHES OF COPPER NUMISMATIC PERSONALITIES (http://www.1794largecents.com/1794/bios.htm)

Robert Coulton Davis' "Pattern and Experimental Issues of the United States Mint" was published in the Coin Collector's Journal, 1885.

The Breen tale about R.C. Davis giving drugs to Mint officials is a classic Breenism. In the 1870s and 1880s anyone at the Mint or anywhere else could have bought opium at a drugstore without prescription. It was not until the 1906 Pure Food and Drugs Act (effective January 1, 1907) that narcotics were restricted. Mrs. Winslow's Soothing Syrup for babies, for example, popular in the 19th century, had opium as a base and was available over the counter.