The following people are found in early Orangeburgh County and Barnwell County, SC census records:

See Joseph Emanuel:
1790 -- household include himself, 1 male under 16 and 2 free white females.  Could be Joseph, John, unk. female & wife?

See John Manuel (ca 1790s - 1840s):
1810 -- only shows 2 boys ages 10-15 (John & William ?), 1 female over 45, and 1 slave. {perhaps the father of John and William}
1820
1830a1830b -- age 30-40.  John has a girl under age 5 who could be Mary B. Manuel.  John's wife was born 1791-1800.
1840a1840b -- age 40-50.  John has a girl who could be Mary B. Manuel.  John's wife was born 1791-1800.  He has four sons: births are 1831-35, 1826-30, two 1821-25.
1850  age 75.  Who was this John Manuel (born ca 1775) and his wife Polly (born ca 1790)?
1850 -- age 30.   This must be John Manuel, Jr. (born around 1820).  He was living near William Manuel, age 50.

See William Manuel (ca 1800 - 1850s):
1830a1830b -- age 30-40  William's youngest girl is age 5-10, and so probably is not Mary B. Manuel.  William's wife was born 1791-1800.
1840a1840b -- age 30-40.  William has a girl who could be Mary B. Manuel.  William's wife was born 1791-1800.
1850 -- age 50.  He was living near John Manuel (Jr. ?), age 30.  William's son, William (born 1832) is still at home.
1850  -- age 25.  This must be William Manuel, Jr. (born around 1825)

Hypothesis:

In 1790 Joseph Emanuel is husband to an unknown wife, has a young boy (born between 1775-1790) who may have been John, and has another female of  unknown age in the household.
In 1810 there is a "John Manuel" household, with two boys born between 1795-1800 and one female born before 1765.
In 1820 no Manuels are found in Barnwell county census records.
In 1830 John and William both have young girls.  Either girl could be Mary B. Manuel, but John's girl's age is more consistent with later data for Mary.  The ages of John and William's wives are within 10 years of each other.  There are no elderly women in either household.
The data of 1840 do not alter the conclusions from 1830.
By 1850 John has died, but William is still living and has a son William (born ca 1832)  William's wife Affy was born around 1802.
In 1860 Eva Manuel (born ca 1793) is listed living with William Manuel (born ca 1833), Lilly? Harvey (born ca 1821), {female}Gale (born ca 1836) and her grandson Virgil Manuel (born 1849).  Eva is most likely the grandmother or greatgrandmother of these people.  Mary B. Manuel (born 1829) is proved mother of Virgil Manuel (by his death certificate). Mary was most likely a daughter of  John Manuel (ca 1790s - 1840s), who had 3 daughters; the records of 1830, 1840 and 1850 show that William Manuel (ca 1805 - >1850) had only 1 daughter.  Eva appears to have been the widow of John Manuel and mother of Mary B. Manuel.
Question:  Where was Eva Manuel in 1850?  Who is the John Manuel born ca 1775?