Hi, The oldest Census I found on Amanda M. Smith was the 1900 when she, Robert and children were living in East Challam, Clallam Co., WA. Amanda M. Smith, b. TX April 1875. I assume you have your children's names so won't list them.
In 1910 she, Robert and family were living in Florence, Lane Co., OR.
In 1920 she, robert and family were living in Sixes, Curry Co., OR. This is the last Census that Robert appears on. His middle name is McKay. I'd contact a lookup volunteer at Random acts Genealogical Kindness. Go to raogk.org, read guidelines, choose oregon, choose ALL for County and choose Mabel who does look ups in the Oregon Death Index 1903-2005.
When you find his death date, you can request an obituary for him through RAOGK but enter Curry for name of County. I'm hoping his obituary will say something about when and where he married Amanda. Then you can pursue their marriage record which will contain her maiden name and hopefully where she was born.
In 1930 Amanda and some children were living in Port orford, Curry Co., OR.
I'd go back to RAOGK and ask Mabel to look up her death date. Then find her obituary and hopefully her maiden name will be on it. Successful Searching!