The movie was shot at Shimer College in Mount Carroll, Illinois. That campus is now the home of the Campbell Center for Historical Preservation that educates museum curators from around the world.
In the scene where the Colonel is driving his car across the bridge on his way to his destination; that is the bridge that connects Savanna, Illinois with Sabula, Iowa. He is actually driving the opposite direction (West) needed to get to the filming location (East).
This movie was produced in part as a pilot for a prospective television series, which is consistent with Colonel Knox suggesting in the abrupt ending that he and Top come back the following Monday (beyond his alloted two weeks).
At 18:06 minutes into the movie when Hard Knox goes to visit General Garfield the location used as the "Sunnyside Convolescent Home' is actually the Caroline Mark Home a historic retirement home for 'women only' located at 222 East Lincoln Street in Mount Carroll, Illinois. The home was built in 1906 through an endowment made in Caroline Mark's estate. Mark, who died in 1900, and her husband James were longtime Mount Carroll residents. Caroline Mark bequeathed her estate to provide housing for the area's elderly women, who were often poor and had little to no social support. The home was built and operated using the funds from her estate and is still funded by the income provided by Mark's real estate holdings. The home was added to the National Register of Historic Places on August 11, 1983
Bill Erwin the actor playing 'General Garfield' was born in 1914 and at the time of principal photography was 69 years old vs. the age of 87 years old he tells 'Hard Knox's' character during their visit at the Sunnyvale Convolescent Home.