We did a cruise on Princess over Thanksgiving about 6-7 years ago. They had a "traditional" menu in the dining room (turkey, stuffing, etc.), but still had other things to choose from. Haven't cruised Royal Caribbean over Thanksgiving, but would be very surprised if they DIDN'T have a traditional turkey dinner on their menu.
We didn't enjoy the Princess cruise because there were over 800 children on board. Kids constantly running up and down the halls all hours of the day (and night). They were switching room service breakfast orders from one doorknob to another, there was food in all the hallways and in some public areas, kids were sprawled out on the stairways, etc.
We have two kids of our own, so we're not "kid haters". But, 800 at one time is a little overwhelming.
Can't say what the passenger profile might be, but since it would be over a holiday, there will most likely be more families. Also, you're likely to find a younger crowd on a short cruise. You get into the 10+ day cruises, then the "geezer count" goes way up.