The term "the truth" in JWism may be a carryover from Freemasonary. Though with the Masons it is the pursuit of truth, not that they are in "the truth" as the JWs define their religion.
From what I've read on this forum, the use of "the truth" or "in the truth" started in the 40s or 50s. By that point the JWs were pretty distinct from any potential freemason influences. My theory is that this is just a meme - an idea that someone had up top because it worked for Jesus ("I am the way, the truth and the life") and it stuck because it was helpful at keeping people indoctrinated and separate from other people. Probably a pretty similar process to what develops most of the loaded language in JWism (and other cults).
Personally, the term makes me sick to hear it used in reference to the cult. I haven't used it since I woke up, and I think going back to exclusively using the JW loaded language words as they're used by the normal populous is a very useful part of cleaning out lingering indoctrination and any reservations/doubts one might have about leaving. It's a very unconscious part of their control, and it's difficult not to be influenced by it. As Steve Hassan said, words are the tools we use to think. If, upon leaving the cult, you continue to use the loaded language of the cult, your thinking will still be influenced by your past indoctrination.