Trump's appeared on the Howard Stern show a zillion times. Being a pussy hound has been a well known aspect of his personality for decades. This particular revelation probably had the blockbuster effect it had because (1) he was declining in the polls and showed no real signs of reversal, (2) Trump refused all reasonable advice and guidance, like how to prep for debates, where to focus his assets and efforts, etc (I mean why in the world was he holding rallies in solid red states he was going to win anyway) and (3) Trump was uncontrollable. The RNC and other GOP politicians saw a hopeless case that was just going to drag the rest of them down and they finally bailed.
I don't blame the RNC for not vetting presidential candidates, that's not their job-it's the job of primary voters and their primary opponents. Trump proved to be Mr. Teflon until a month before the finish line, now he's going to go into full crazy uncle mode. No way he will quit-not with his ego, and no way the GOP could sack him-people have already voted, the absentee ballots are already out to military personnel and others, and most states have laws prohibiting changes in the official ballots at this late stage. The RNC will put all their efforts into holding the Senate from here on out.