Carnival's Elegant nights are more casual. You can dress up if you please, but more casual shirts and slacks for men is acceptable.
I wouldn't worry about leaving luggage unlocked. If you've packed something you shouldn't in your checked bag they'll hold the bag rather than deliver it.
Carnival does a good job, and you'll enjoy the cruise. I personally find Carnival's food better than RCI. The service will possibly be a bit more relaxed than you've experienced on RCI.