I noticed that the Nazi propaganda department was literally called "Reichspropaganda-Leitung (RPL)", "the propaganda office". Was this the public name of it, or, a secret name used behind closed doors?
The Reichspropagandalleitung (RPL) was not a state agency but an organization of the NSDAP and was officially independent of the Reich Ministry of Propaganda.
The main task of the RPL was to monitor, coordinate and unify the propaganda of the NSDAP, its branches and affiliated associations. Its special focus was on broadcasting.
And of course the people in Germany knew that the German media, such as newspapers and radio, were controlled by the NSDAP and the Reich Ministry of Propaganda. Thus, especially during the war, people always took advantage of the opportunity to listen to the broadcasts of neutral Switzerland or the Allies, at the greatest risk to life and limb.
Yes, but I think you got it the wrong way around:
They all knew that there was an office of propaganda. It was literally broadcast on the only radio channel. It was their name and totally normal.
But back then "Propaganda" did not have the negative connotations it has today. If you look at the word itself, "to propagate" there is nothing negative about it. It only became a negative word because the fascist regimes used it as a word in their vocabulary and it became a synonym of their information war.
The ministries of propaganda still exist today in every single country. Sure, it's called press secretary or public relations now. But it's the same business. They just do no longer call it propaganda, because that's the word the fascists used.