I agree that often to know who is who you have to know the wife and that can be difficult too because the mindset of a lot of people is that the male (surname) is the most important line. I have been researching for almost 40 years and that is so far from the truth!
As I said I am not related to the Rankins, my husband's gg-grandmother is Lucinda Black and sister to Annie Rozella Black. In your first post you mention that your early ancestor may have married a Sarah Landrum but you are not sure. Lucinda Blacks husband was Granville Landrum McLin, and Robert Rankin and Annie named a son Landrum. Interesting I think.
I have briefly looked at census records and the first time I find Robert W Rankin is the 1880 census in Montgomery Co., TX. He and Annie have 2 young children in 1880. I believe they are in Jasper Co., TX. by 1900. Robert died about 1905 and Annie in 1935.