I think this is correct in most hotels. The Front Desk isn't involved in this aspect of hotel operations, so they wouldn't necessarily know where to find that information in the system.

While your suggestion of contacting a Sales Manager would probably work (as would an on-site Reservations or Revenue Manager), I'd vote for contacting person/office in the company who did negotiate that rate. To me, that's easier than trying to track down someone at the hotel.