Sometimes people from this region reported their origin as Schleswig, sometimes Holstein, sometimes Prussia. It can be a nightmare.
In one census (Wisconsin state, 1905) my Schleswig Petersens were enumerated as being from Norway!
You may have to look at documents from the USA to narrow it down. Have you looked into death certificates for your folks? Sometimes the town of origin was indicated.
Do you know their emigration date? Many Schleswig-Holstein emigrants left from Hamburg. Ancestry.com has Hamburg departures in their database in German but readable. Often the last place of residence was recorded, which will give you a clue to where they lived, at least.