Accessible Here. It's called "The Parts that were Left out of the Kennedy Book" and features among its excerpts this:
"Of course, President Johnson is often given to inappropriate responses--witness the puzzled timing of his smiles when he speaks of grave matters--but we must also assume that Mrs. Kennedy had been traumatized that day and and her perception was likely to have been colored by the tragedy. This state of shock must have underlain an incident on Air Force One which this writer conceives to be delirium, but which Mrs. Kennedy insists she actually saw. "I'm telling you this for the historical records," she said,"so that people a hundred years from now will know what I had to go through."
"She corroborated Gore Vidal's story, continuing: "That man was crouching over the corpse, no longer chuckling but breathing hard and moving his body rhythmically. At first I thought he must be performing some mysterious symbolic rite he'd learned from Mexicans or Indians as a boy. And then i realized--there is only one way to say this--he was literally fucking my husband in the throat. He reached a climax and dismounted. I froze. The next thing I remember, he was being sworn in as the new President."