Will Book 1, p.86, Roanoke Co., Salem, Virginia, dated 22 Jul 1853, probated 21 Aug 1854. Names wife Sarah, son Nicholas and daughters Catherine Grill, Mary Fullhart, Anna Fullhart, Magdalena Christ, Elizabeth Trout, Sally Gerst and Barbara Howard.
Early Clark County Families, Vol.2, pp. 169-170 contain reader-submitted corrections and additions to the family of Michael Frantz Jr. and Magdalena Zug. One submission (by Thomas J. Beardsley) has Peter’s birthdate as "ca 1757", another (by Lowell Beachler) has it as 1765, and a third (by Lela Shoup) has it as "ca 1764 VA".
This family is listed in Our Garst Family
7 on p. 28.
The surname is probably a married name from a previous marriage. My source for this is "Our Garst Family", p. 28, which says "Records show that Peter Frantz later married Mrs. Sarah Pleasants."