The term likely goes back to Russell and the 19th-century grass-roots Protestantism that he moved around in theologically, plenty of those Adventist types, dispensationalists, and other fundamentalists who were all caught up in the fulfillment of biblical prophecy. They liked the term "present truth," which is defined as "those scriptures of particular relevance to the present dispensation," (J. Gordon Melton, A Dictionary of Religious Bodies in the United States, 1977, p. 249).
From that, most likely under Freddie Franz's direction, it morphed simply into "the truth," connoting whatever doctrine he put forth, gobbled up so eagerly by the ever "faithful" rank and file with their obsession on "unity at all costs."
Presto: "the truth!" Those books you cite, Ron.W., just reflect this history.