I note that your grandmother, Mary Abigail Johnson, born in 1857 and died in 1921 is buried in section A, Lot 40, Location N 1/2. But, this you probably already know. I do not find Rolland A. Johnson buried in Custer County Cemetery, although I find in other sources that he died October 23, 1937 at age 79 in Miles City, Index #2349.
Of interest, I do see that another Mary Johnson, died May 21, 1924 and is buried in Section B, lot 65, location W 1/2, Alley LT. Under the comments section is noted "Remains Mr. Johnson". I'm not saying that is your Grandfather Mr. Johnson, but it would bear some research to see if they are indeed related. If that Mary Johnson did not have a husband, then perhaps it is your grandfather?????? I'm just speculating here and "leaving no leaf unturned" so to speak.