I've just finished the book closing with the second Save Star Trek campaign. Cushman acknowledges the descrepencies in how many letters NBC may have eventually received. Regardless of the final number the campaign evidently made enough of an impression to encourage NBC to change its mind and continue for another season. That said Cushman does allude that the seeds for the show's eventual demise were already being sown.
It's ironic, really. TOS got soft ratings in second season because of its bad timeslot,
It's interesting to note that some affiliates and other markets (like Canada) got better number because they aired the show earlier in the week, something NBC seemed happy to overlook.
One thing I found really weird is how they scheduled TV shows back then. Who would put an hour long show starting on the half hour? It seems like a really odd choice.
-Chris