Option 1: Take train till Ahmedabad and Bus (Volvo/Rajasthan Roadways) from der.
Option 2: Try 12961 Avantika till Ratlam; Catch 19653 from Ratlam for Chittorgarh; From Chittorgarh go to Udaipur by Bus/Train
Option 3: Check 22901 in sleeper till Ratlam, in case you get ticket, take waiting for Ratlam to Udaipur.