That was a rumor spread by a campaign consultant whose firm worked for McCain. I'm not aware that Romney ever attacked attacked Palin himself, and there was enough backbiting from McCain loyalists against Palin that you don't have to hypothesize any Romneyite conspiracy behind the scenes (plus, I'm pretty sure that if Romney were involved, greed wouldn't have been a motive with all the money he has).
Much of the campaign staff assigned to Palin came from Romney staffers who hired onto the McCain campaign after Romney conceded defeat in the primaries.
Their interest in demeaning Palin was to remove her from consideration and keep the field clear for Romney in 2016. Whether they were acting on their own -- or with Romney's blessing -- is uncertain.