Yes Schultz knew. In many shows he witnessed the bunk open to the tunnels and said nothing.
He helped pull off some missions, like One to get Newkirk and carter back after they were captured. Or the one where they had to get a replacement painting of the “blue boy”. It suspected he was tired of war (after fighting in WW I) and was anti Nazi due to them taking his factory for the war effort. Now Klink did he know possibly but because he had a no escape record he did not go to the russian front. There was several times he need a train to blow up and it did after Hogan convinced him it would, then ther was the “Super” sergent he asked Hogan to get rid of, and the one time he was sent to the russian front and Hogan and the other prisoners boarded the the train to pull his bacon out the fire . Did he know? I think he did not really care as long as he stayed in Stalag 13.
He could have “retired” from the German military after WWI (i.e., was likely made redundant) and worked as a civilian during the interwar period. He would then have been called up to serve again as the Third Reich was preparing for WWII.
I came in to mention precisely this. Per Shultz’s reminiscing description of himself as a “big wheel” and the owner of an assumedly substantial factory he was evidently (pre-war) an executive and a fairly wealthy and influential man which is at odds with his role as a clueless buffoon. This is the hardest canon evidence you will find that Schultz was putting on an act.
I ran out of time but per my last post but it’s fascinating how many of the actors in that show were directly involved in the war, some in very substantial ways. WWII seems so far away now, but when that show aired in 1965 it was only 21 years after the war. If you served in WWII in your 20’s many actors would have only been in their 40’s at that point.