Here are some suggestions for you.
Attach all the records you are SURE of to your family tree on Ancestry. That will help you follow the family from one place to another. You should eventually try to identify everybody in all those records.
From your query, it appears that you believe the 1850 census record in Perry Co. IL is your correct family: John 33, Mary 31 and 5 children.
You also indicate that the same John Renfro is the one living in 1860 in Franklin Co. IL (Methodist Clergy). In that record his wife is apparently Sarah. The only child with the same name is Susan, 13. So you will need to reconcile the differences in the children between these two censuses. There are two children with the name Long: Euphrates 13 and Martha 8. This suggests that Sarah is the mother of these two children and that she was previously married to a Long.
Sure enough the 1850 census for Franklin Co. shows Sarah as the wife of Jas. M. Long. Euphrates is in the household.
So at this point the best guess is that Mary died between 1850 and 1860 and John Renfro married the widow Sarah Long. This is a theory only at this point. You will have to confirm or refute this theory with other records.
I also located John M. Rentfro in 1870 in Jasper Co. MO. His wife was Sarah. They apparently had a son George C. 8. Also Martha Long was in the household. Note that there was a William Rentfro age 54, b. TN living next door. It is possible that this is a relative.
In 1880 Jno N. Rentfro was still living in Jasper Co. MO. He was a minister and farmer. His parents were born in VA.
You have apparently found the entry on find-a-grave for J N. Rentfro, buried in Jasper Co. MO. Please note that the reference to "Margaret" is a comment made by someone. At this point you have no information as to whether this is correct, whether that was Sarah's middle name, or what. The person who left this comment did not explain where he/she got the information.
Finally, here is a link to Rentfro marriages in Illinois. Note that a John N. Rentfro married Mary Ann Bibb in 1842 and Sarah Long in 1859, both in Franklin County.
I think after you attach and account for all possible records for this family in your family tree, you will have a good timeline from which to proceed further.
Good luck with your research.