Just got a new 850I, gas cap leaked bad even with half tank sloshing around. I put another gasket on the cap ( total of 2) on the cap and this fixed the leak. Evidently something about the threads does not allow just the thickness of 1 gasket to seal.
Two possibilities... you got the wrong cap... which did happen at the factory on the XUV550's... One of the caps have a small hole in the top and the other does not... I forget which ones is right for RSX. Also you might have a defective cap... your dealer should take care of it.