Erin Moran died purely from complications of advanced cancer ... according to the Harrison County Coroner, whose final autopsy results confirmed what Erin's husband had said.

The coroner's report says, "no illegal narcotics were involved in her death" ... and she was stage 4.

Erin's huband, Steve Fleischmann, had written an open letter describing Erin's months-long battle with squamous cell carcinoma.

The "Happy Days" and "Joanie Loves Chachi" star died last month, at the age of 56.

Steve and Erin were together for 25 years. He says she died holding his hand.