First.. I can't answer your question because I don't know for sure.
But I can say that it may depend on the cabin you are assigned. If you already have an assigned cabin, check to see if it will hold 2 people (although there can't be too many that would not IF there are any single cabins on ships any more)
But, I don't see why they could not add a person to your cabin. I don't think your travel agent is being honest. It would be more work for them, but I don't see why it can't be done. You could always call RCCL and asked them if it can be done. If they say yes, call your TA back and tell them to make it happen. Expect to pay more - basically the second person fare for the additional person. They may or may not refund the difference in your single passenger rate and the "based on double occupancy rate" or they may not. But the single rate is not a full 2 person fare, so putting a person in the cabin with you won't be "free"
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013