Hi Colleen
Robert's birth or marriage OPRs might clarify his mother's name.
Robert's place of death is Moniaive, Dumfriesshire. The Dumfriesshire censuses will indicate where he was born. I can see a Robert Sloan on the 1861 census, aged 60, living in the Parish of Glencairn, Dumfriesshire, born Kirkcudbrightshire. I think it's the same Robert on the 1841 census, aged 40, but his birthplace is given as Dumfriesshire there.
https://familysearch.org/pal:/MM9.1.1/VBQ1-ZRShttps://familysearch.org/pal:/MM9.1.1/VYD9-RFRIs it possible Robert was his middle name and he was baptised under a different name? Or, did he have a brother called William? On SP births there's a William Sloan, son of James Sloan and EUNICE MOORE, born in Kirkcudbrightshire, July 1799.

On FS, I can see various christenings of children of Robert Sloan & Ann Clelland - Samuel (1825), Marion (1829), and James (1831) - all born at Moniaive. So his marriage was probably after 1819 (aged 18+) and before 1826. There are various Ann Cleland marriages on SP for that period, but none to a Sloan. Is it possible Ann married before, was widowed, and then remarried Robert under a different name?
Failing that, did your Robert have any known siblings (e.g. witnesses at any of his children's births or marriages)? The siblings' BMDs might have clearer details of the mother's name.