If you are Platinum or Diamond members with RCCL, the cabin attendant will supply robes, Platinum level is 5-9 past cruises, and Diamond is 10+ cruises on Royal Caribbean Cruise line. If you don't know your level, you can call the Crown & Anchor Society at 1-800-401-9370.
I haven't been on the Rhapsody, but when I was on the Vision (Rhapsody's sister ship) this past January, I stayed in a category D cabin. No robes were provided. However, I asked my cabin steward for some robes and they were waiting for us in our room that evening.
Our travelling companions staying in a regular outside cabin also asked their cabin steward for robes, and they received them for use during the cruise also.
Therefore, if they aren't in your cabin, ask your steward and you'll probably get them.