"RD" was a graffito left on the palace of Versailles by the Royal Drive gang when they stole the palace's baroque throne room (and somewhat less baroque antechamber). They left equivalent graffiti, though in the local written languages, on similarly ransacked palaces on Amana XI and Alpharalpha B. This was the only clue Space Patrol Cadet Rufus Q Shupilluliumash was able to find on these planets before being ejected by the respective local authorities of each planet. He attempted to Google the letters' meaning, but was not very thorough; he hit the "I'm Feeling Lucky" button, found irrelevant information on Research and Development, and called it a day.
On Gould IV, the graffiti was rather longer. It read "Next Stop--Galactic Central! Royal Drive." From this, Rufus cleverly deduced that "RD" meant "Royal Drive," and that the throne room thiefs would next stop at Galactic Central.