"Whistlin' Willy's Pizza Gultch" is not a real restaurant but is believed to be a parody of the restaurant "Chuck E. Cheese's," http://www.chuckecheese.com. which are located all throughout the United States.
As Eric Cartman says, there is no "Pizza Hut" in South Park, Colorado, United States. However, "Pizza Hut" is a real restaurant, http://www.pizzahut.com, which are located all throughout the United States.
"Colomax," "Colomax PM," Colomax: Daytime Cold and Flu." and "Colomax: Maxium Strength Cold and Testant Cough and Cold" are indead fictional cough medicines.