[Ger-Poland-Volhynia] Kaminski ethnicity

[Hal and Jan Kamm]

This is my first post on this list.

I am a grandson of Arthur Kaminski, who arrived in US in 1907 listed  
as Russian nationality, but German ethnicity. The ship manifest noted  
Ripen as the town of origin. I cannot find a town named Ripen, but  
there is one called Rypin in Kujawsko-Pomorskie, Poland.

I am confused about the German ethnicity versus Russian/Polish  
nationality. I had heard stories years ago about the family working  
as millers of grain across northern Europe. Would names be changed  
temporarily, during one generation or less, depending on where they  
were living?

Any help would be appreciated.
Thanks