It was while in New Orleans--presumably living with his family--when Robert Barbour Johnson agreed to be a press agent for a traveling circus, according to his memoir in The Weird Tale Story.  As he recounted there, he passed through most of Canada and the U.S. during his summer stints (he was still enrolled in Tulane otherwise).  Quickly bored by his duties, he "discovered a new talent, that of animal training.  I was soon handling dogs, ponies, goats, horses, and even camels."

Whether RBJ actually worked for the circus is difficult to know.  Circuses performers are notoriously hard to track--circuses are where people run away to, where they go to get lost.  There's no special reason to disbelieve Johnson, and some things which give credence to his story.  He wrote about circuses often for the pulpish magazine Blue Book throughout the 1940s and 1950s, as wel as a few articles for some smaller magazines.  Here's a probably incomplete bibliography:

# The Big Hitch (ss) Blue Book Feb 1948
# Animal Man (ss) Blue Book Apr 1948
# Elephant Boss (nv) Blue Book Aug 1948
# White Horse Pioneers (ss) Blue Book Nov 1948
# Lion-Tamer (ss) Blue Book Dec 1948
# Parade Wagon (nv) Blue Book Feb 1949
# The Man Who Didn’t Like Dogs (ss) Blue Book Apr 1949
# Buffalo Bill Holds Five Kings (ss) Blue Book Jun 1949
# Truck Show Bull (ss) Blue Book Aug 1949
# Liberty Horse (nv) Blue Book Dec 1949
# Grift Show Parade (nv) Blue Book Apr 1950
# The Big Herd (ss) Blue Book Jul 1950
# Zebra Team (ss) Blue Book Dec 1950
# King of the Cage (ss) Blue Book May 1951
# Longneck (ss) Blue Book Jul 1951
# John Robinson Rides Four (ss) Blue Book Dec 1951
# Killer Lion (ss) Short Stories Dec 1958
# The Clown (ss) Short Stories for Men Aug 1959

He was also well known for training his pet cat, "Kitty K."  Clark Ashton Smith's wife, Carol, commented on it.  And at the 1956 West Coast Science Fiction Conference (Westercon), Johnson's pet performed--in the words of the Oakland Tribune--35 stunts you wouldn't normally expect a cat to do.  Unless it was Jueles Verne's cat."

But even if--and it's only a slight if--Johnson never joined the circus but only invented these tales, it is nonetheless true that the circus world meshed with his continued interest in the fantastic.  He met someone, he said, for example, who planned to hunt for the Sasquatch at a time when the creature was not well known outside the confines of Canada.  And in a world of actual freaks, the weird stories he favored seemed a lot more plausible.