On a 7 night cruise with most cruise lines the 2nd night on board is usually the first formal night and the 6th night (or second from the last which would be casual because you have to pack) is the sendond formal nigt. The old cruise joke is to wear your smallest formal outfit first as it ay not fit by the end of the week. RCI will probably have 2 Informal nights or sometimes it is called Smart Casual, which really confuses people. For gentlemen this usually means blazer and tiewit slack & shirt and ladies a nice dress or pants suit. Your brochure which should come with your doc's any day, should list these facts, too.
My wife and I just got off the EOS saterday and on the 2 formal nights about half of the men had tux. - but a suit and tie is ok , you will have a great time, remember it is a great resort hotel that happens to move to different ports