I only have time for a brief response. The average American would have certainly known that the Nazi regime persecuted Jews. However, they almost certainly would NOT have known the extent and extremity of the persecution (at least until after the camps were liberated, and contemporary research shows that we're still not finished understanding the extent). The general reaction of Allied troops who liberated the camps was shocked disbelief. People generally understood that wartime propaganda is filled with exaggeration. In this case however, the propaganda was actually restrained and understated compared to the reality of what had actually occurred. General Eisenhower was notably concerned with ensuring that he was in a position to bear witness to something that was, quite literally, unbelievable.

"I visited every nook and cranny of the camp because I felt it my duty to be in a position from then on to testify at first hand about these things in case there ever grew up at home the belief or assumption that `the stories of Nazi brutality were just propaganda.' Some members of the visiting party were unable to go through the ordeal. I not only did so but as soon as I returned to Patton's headquarters that evening I sent communications to both Washington and London, urging the two governments to send instantly to Germany a random group of newspaper editors and representative groups from the national legislatures. I felt that the evidence should be immediately placed before the American and British publics in a fashion that would leave no room for cynical doubt."