In American English "I became a teetotaler" sounds as natural to to me as "I became a vegetarian." "Teetotal" sounds entirely wrong. In fact, this is the first time I remember having encountered that word, although I did verify just now that it is in the Merriam-Webster Online Dictionary.The only time I have heard the word teetotal is in the following cases:
I am going teetotal
I am teetotal
I have been teetotal for the past X years.
I have never heard it being used as becoming a teetotaler. It doesn't sound right to me.