I like the idea that the JWs call God by a nickname.
In any case, "The Truth" is a broad and context-dependent term. When we refer to the truth of a matter, we are likely referring to known facts. But we can also use the word to refer to how we interpret a situation, such as the classic "you know it's the truth," used to try to convince someone to see things a different way.
JWs are claiming that they have the correct facts about God- who He is, what He wants, Why he wants it, How he intends to act, and so on. In this, they're like any other religion. They do seem to really harp on it, though. I think it is another of those mantras that they want the rank-and-file to keep repeating, so that it sinks in ever deeper. It's another way to isolate people- if you are convinced that the JWs have the truth, then you have nowhere else to go.