Name: Benjamin Albert Davis Death Date: 13 Dec 1949 Death Place: Riverside, Walker, Texas Gender: Male Race: White Death Age: 67 years 1 month 7 days Estimated Birth Date: Birth Date: 06 Nov 1882 Birthplace: Texas Marital Status: Married Spouse's Name: Father's Name: Eck Davis Father's Birthplace: Texas Mother's Name: Lou Ruthledge Mother's Birthplace: Texas Occupation: Farmer Place of Residence: Riverside, Walker, Texas Cemetery: Riverside, Texas Burial Place: Burial Date: 14 Dec 1949 Additional Relatives: X Film Number: 2223549 Digital Film Number: 4031938 Image Number: 458 Reference Number: 61428
And if you go to Find a Grave you will see double headstone for the two of them. PLUS photo of headstone for Mattie, too.
You might check for divorce record in county where Robert and Martha last lived together. You never know what will show up in those records.