After doing a bit of looking online I think that Gustav E Moldenhauer's parents are Gustav and Hulda Moldenhauer.
The 1910 census has them in Albon Tw. Grant County. Gust is age 44 immigrated 1891. Hulda is 43. With them are Gust E. age 20 born Germany, Emil age 5 born SD, "Tilo" age 3 born SD.
In the 1920 census Gus Sr., Hulda are with "Amelia" age 15, and Tilo age 13 in Vernon Grant Co.
In the 1930 census Gust Sr. is with Hulda, Telo age 23.
Hulda Moldenhauer arrived in NY Mar 1892 with Gustav age 2 on the ship Oldenberg. Their destination was South Dakota.
Findagrave shows Gustav E., Lenora and several other Muldenhauer buried in the Milband Cemetery Grant County SD. Earl A.1914-1993 is listed as the son of Gust and Lenora. Darwin is the son of Emil and Erna and includes a complete obit. Norman is the son of Telo and Selma and also includes and obit.
There are a number of trees on Ancestry that agree with what I have listed above.
Gustav Moldenhauer appears to be a suprisingly comman name.