Zhupan is an old title of sovereignty and means something like duke or prince, though I don't know if it can be translated with 100% accuracy. Stefan Nemanya was the Grand Zhupan who founded the first lasting Serbian state in the 12th century. He succeded in establishing sovereignty from the Byzantine Empire in 1185, if my memory serves me correctly. It's interesting that Nemanya was baptized in a Roman Catholic church and later became Orthodox. I would love to know more about why this happened, may try to ask why on a Serbian forum. Maybe some Serbs wer RC in his day, but Orthodoxy became the established faith and proper orthopraxis received encouragement from Nemanya's son St. Sava, who established the autocephalous Serbian church. Note that Nemanya himself eventually abdicated and became a monk under the name Simeon. His relics let forth myrrh and he is known as Sveti Simeon Mirotochivi (St. Symeon the Myrrh Gusher).
Later on, many of the Serbs who lived in Dalmatia were Roman Catholics, I am guessing because of their Venetian and/or Austrian overlords. Later, these Serbs were assimilated to the Croatian population and indeed much of Dalmatia is now part of Croatia, officially (it wasn't in the past, though). And during World War II, when Croatia was a Nazi puppet state, Serbs faced horrendous persecution there. They were mutilated and killed in such abominable conditions that the Germans were ashamed of it. Some Serbs formally converted to Roman Catholicism in face of this persecution. I think there were mass ceremonies when they would gather a lot of Serbs and bring priests to "convert" them. I don't know how many, if any Serbs did convert during the last wars, but as Croatia was ethnically cleansed of scores of Serbs, I don't know if there are many people remaining.