I have never travelled with RCC but I expect the service is much the same as elsewhere. If you make your requirements known they will be met. There's always loads of fish and salad on the buffets. Low fat spreads and yoghurts are also available. Don't worry about it and enjoy your holiday.
The main restaurants offer healthy low fat, low cholesterol options, vegetarian choices, ethnic and regional specialities, as well as traditional cuisine, so you should be well looked after.
Yes they offer healthy low fat food in the main resturant, the buffet has alot of different selection including a very good salad bar. Also if you let know they will try their best to sort something out. You could email Royal Caribbean with the information of your cruise and let them know and they will suggest what to do, they may even get a note put on the system so your waiter already knows. They are usually very helpful, if emailing via their web it may take 1 or 2 days to get a response, but they do respond!