Levi Johnston, the father of former Republican Vice Presidential candidate Sarah Palin's grandson, is going on the record about what he knows about Palin and her family.

Johnston, who welcomed son Tripp almost a year ago with Palin's teen daughter Bristol, tells Mark Steines that the public Palin is very different from the private one: "The TV side of Sarah Palin which I am telling you is not the real Palin. She goes out and puts on a big show for you and that is the Palin you know."

He explained that Palin used some surprising terms to describe her son Trig, born with Down Syndrome. Johnston says, "She'd come in from work and she'd be like, 'Hey, where is my retarded baby,' and just something like along those lines. The kids, Bristol and them, didn't think much of it, but the first time I heard it, I was in shock, I didn't know what to say. So, but then she, it kind of became a normal thing for her to say and you know, I guess, but she'd always have a smile on her face, laughing about it."

Palin, for her part, seems to deny ever saying such a thing in a statement she released to ET, saying, in part, "We have purposefully ignored the mean spirited, malicious and untrue attacks on our family. We, like many, are appalled at the inflammatory statements being made or implied. Trig is our 'blessed little angel' who knows it and is lovingly called that every day of his life. Even the thought that anyone would refer to Trig by any disparaging name is sickening and sad."

Johnston tells Mark that he keeps up on his child support for son Tripp but that he doesn't get to see him as much as he'd like, and that he'd eventually like to share custody equally.

Watch ET for more shocking revelations from our interview.