Does not exist. Popular fictionalized location found in multiple horror and pornographic novels.
Books mentioning Knoxville Iowa:

One stone, two stone, three stone scream! : A Urological Nightmare by Harlin Emerson

Bambi's Adventures on Backwards Day by Ruby Goturnose

The People Under the Portapotty
By Semore Butts
by Papadoc June 20, 2014
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