I am pretty sure it was at the St. Louis Arena.
The St. Louis Arena was much bigger than the Kiel Auditorium (the arena could seat 18,000 and the auditorium could seat 9,000) and by that time Led Zeppelin were playing massive sports arenas and stadiums. Besides, IMO it wouldn't make sense to play for 16,000 people in Tuscaloosa, Alabama (with a population of not even 100,000) but only play for 9,000 people in St. Louis, which has a population of 309,000.
It could've have been moved from the Kiel Auditorium to the St. Louis Arena for extra tickets to be sold.