In this passage, the 1837 edition changed where to whence, apparently a typo since whence means ‘from where’, which obviously will not work here (although this may not have been obvious when the 1837 change was made, since whence was by then archaic in English). The 1908 RLDS edition restored the original where to the RLDS text (probably by reference to 𝓟); the 1920 LDS text made the change in the LDS text (probably by reference to the 1830 edition or for grammatical reasons).
The Oxford English Dictionary points out that from whence is a redundancy (see under whence). Interestingly, the Book of Mormon text has 21 examples of whence and every one of them is preceded by from. On the other hand, the King James Bible has 72 occurrences of whence, of which only 27 of them are preceded by from. Thus we get variation in the King James Bible, as in the following contrastive pair of questions:
But we never get this variation in the original Book of Mormon text; there is only from whence. Thus the secondary reading here in Alma 27:25, “whence they had pitched their tents”, is inconsistent with Book of Mormon usage since there is no preceding from. Of course, if from had been added when the 1837 typesetter changed where to whence, the error would have been obvious: “he returned … into the wilderness from whence they had pitched their tents”.
Summary: Maintain the original where in Alma 27:25 (“he returned … into the wilderness where they had pitched their tents”).