Further research may prove otherwise, but as far as I can tell the first out-of-town training camp in NFL history was conducted by the Milwaukee Badgers in 1922. That year the Badgers prepared for their upcoming season at St. John’s Northwestern Military Academy in Delafield, Wisconsin; about 30-miles west of Milwaukee.
The Badgers opened camp on September 18th, 1922, and trained in Delafield for two weeks. Practicing twice daily under the guidance of Earl Potteiger and Jim Dufft.
Despite a slew of stars (Potteiger, Jimmy Conzelmann, Fritz Pollard, Duke Slater, …) the Badgers went 2-4-3 in 1922, and finished in 11th place.
The Badgers NFL franchise was in existence from 1922 to 1926. Their all-time regular season record was 16-27-6. Their best season occurred in 1923, when they went 7-2-3 and even tied the mighty Chicago Bears late in the season.
For more on the Badgers, I highly recommend that you check out the book The Badgers: Milwaukee’s NFL entry of 1922-1926 by Michael Benter.
St. John’s Military Academy has been a training camp site for two other NFL teams. The Chicago Bears camped there from 1935 to 1943, and it was the Dallas Cowboys’ second-ever training camp site in 1960; a camp best remembered for its rough accommodations.
Erroneously, the Duluth Eskimos are often credited for having the league’s first out-of-town training camp. In 1926, the Eskimos trained in Two Harbors, Minnesota. That same year the Frankfort Yellow Jackets also set up a training camp outside of town, when they trained at the Wildwood Golf Course in Cape May Court House, New Jersey. The Badgers beat both of those teams by four years with their out-of-town camp.