I've done lots of cruises with RCI, and even on formal nights a dress shirt and slacks will be good in the main dining room. This is especially true on the shorter Caribbean cruises Liberty does. On casual nights I usually wear dockers and a golf shirt. The Windjammer buffet is also there for casual dinner every night and they really do put out a high quality spread.
I am going on the Liberty Of The Seas for two back-to-back cruises starting December 3 and don't plan on bringing anything more formal than a sport coat - and I'm only bringing that in case I need it for something unpredictable like a dinner invitation.
I'm back on Liberty in mid-January for the chartered Rock Legends cruise. Those are even better since there is no suggested dress and t-shirts and shorts are worn all the time!