Betsy Johnson of Bergen

I have tried searching Digitalarkivet, but find nothing. My gg-grandmother's obituary says she was born Betsy Ann Johnson on 28 Feb 1850 in Bergin, Norway. She was baptized and confirmed in Norway. She came to America at the age of 18, making her home in Northwood, Worth, Iowa. (All of that is from the obituary.) She is listed on the 1870 census in Northwood as a housekeeper living with the Boyd family. She married Alexander Anderson on 30 May 1874 in Boone County, Iowa. She is listed on the marriage record as Betsy R. Johnson. We have also seen her name listed as Betsy Belle. It seems I should be able to find something in the Norwegian archives, but I find nothing. I realize her surname was probably not Johnson there. Perhaps Johnsdtr? Even checking my dates has come up empty. If anyone has suggestions for what to check next, I would be most appreciative!