"In some German cities you will also find Schlager events...." Very true.
That has definitely been my experience in the last 12 plus years or so, since I started keeping track of this information, such as in Nürnberg, Magdeburg, Soest/Westf., Kiel, Düsseldorf. Bad Sassendorf/Westf...mostly by coincidence when I happened to be in these places while a Volksfest (Nürnberg) or Schützenfest was going on....all a very pleasant surprise, where among the music played includes, fox trot, folk songs, soft rock, marching music, Schlager, etc.
I never heard it in Rüdesheim, maybe since I didn't stick around in the evening.
In Magdeburg in July 2009 it was a Schlager band playing that I heard in an open air event on a Sunday afternoon...a very pleasant surprise hearing Schlager hits from the 1970s to current ones.
I would suggest checking with the Tourist Office or ask the staff at the hotel or see if the hotel has events brochures.