Rabbi Yom Tov Schwarz (See: “EYES TO SEE: Recovering Ethical Torah Principles Lost in the Holocaust”) argues that the Jewish people (led by the Torah observant community) should designate a fast day in remembrance and mourning of the Holocaust and it should be on a day that doesn’t already have a fast associated with other events (e.g., 9th of Av, 10th of Tevet, etc). Have any other Gedolim agreed with this and do any Torah observant communities fast on a day solely in remembrance and mourning of the Holocaust?
In his kiruv 20+ CD series, Rabbi Dovid Orlofsky said, "We observe all of the catastrophic tragedies of our nation on one specific day: Tisha B’Av. The loss of our Temple is the root cause of all our suffering in exile." (If I ever get a hold of Rabbi Orlofsky and obtain permission to publish the manuscript of those incredible shiurim, this response would be in chapter 5 question 8...)
Although lack of evidence is no proof, Rabbi Orlofsky specifically didn't suggest there were any halachic authorities who said to create a separate day for the Holocaust.