OK, let's take it as a given that Sully is right about Levi's cool demeanor in the face of Palin's attacks means he really has something devastating on her, and isn't just a reflection of the sense of invincibility you'd expect from a teenage hockey player.
What the heck could Levi have? It would have to be something tangible; otherwise, it's just a he said/she said situation.
It would have to be something too awful for her devotees to ignore or shrug off; otherwise, it wouldn't be much of a threat to such a talented dissembler as Palin.
And it would have to be something that Levi would have a reason beyond any sympathy for Palin herself to hold back on releasing; otherwise, he'd have let the world know already.
I can think of only one thing off the top of my head that would fit all those criteria, and also tie into Levi's statements that the Palin marriage was a facade: email/text message/photographic proof of Palin making advances towards Levi, and/or of a sexual relationship between the two of them.
Something like that would provide Levi with actual proof, not just hearsay; would kill Palin dead as a political figure; and would be unpleasant at best for Bristol, someone Levi wouldn't want to hurt needlessly.
That theory is, of course, nothing but pure, reckless, unfounded, mean-spirited speculation on my part. But it fits the facts as Sully presents them.