Saints' Herald reports that John Whitmer was reaffirming his testimony in early 1878.
"Editorial Items," Saints' Herald 25, no. 4 (February 15, 1878): 58
Bro. R. F. Hill writes from Cameron, Missouri, of a visit made by Brn. F. W. Curtis, Wm. Cadman and himself, to Kingston and Far West. They saw John Whitmer, one of the eight witnesses to the Book of Mormon, and he assured them that his testimony was still the same concerning that book and the redemption of Zion and the restoration of the Saints to it. He preached there January 13th, the first time since June, 1838.