I use Dreams for my
Disney Cruise vacations.

With that said, they did not offer any on board credit for
Royal Caribbean at the time I booked my cruise.
From my understanding, most Travel agents can not be mentioned on the Dis. So your best bet is to do a google search for on board credit and Royal Caribbean. I found a highly rated Web based agency that offered me a very nice OBC for my next cruise and I've spoken to my agent on the phone several times and am very happy with them so far.
I guess I'm kind of jumping to conclusions that you'd mostly be interested in OBC.

But either way a google search should help you out.
Ron