I used the Royal Caribbean promo for several times in the past years.
During the check in, some agents will tell you it is ok to use the credit at any MGM restaurants, some agents will say it is on property only.
As far as I can tell, whatever these agents said does not matter. If you use the credit towards places in the hotel you are staying, it is normally removed automatically. If you use other MGM properties, it is a crap shot. However, I was always able to call customer service to get the credit applied after checkout.
Not sure if things have changed recently.