I see the one you're speaking of:
MacKford, Green Lake, WI
Charles Prochnow 56
Louise Prochnow 50
Irwin (or Irvin) Prochnow 29
Mildred Prochnow 18
William Naratzka 79
The page further shows that he came to the US in 1878. If you don't have paid access to the other censuses, you can access the 1880 for free
here.
...unfortunately I don't find him there...which means they either missed him, or butchered his name so bad that even a soundex search does not reveal it... I couldn't find him in the 1900-1920 censuses either.
Do you have access to the other censuses? Since we know that he was Charles' father in law, you should also check the 1880 for Prochnow's...and browse the pages near where you find them - looking for William. It's a bit of a long-shot...but not too unreasonable to think that by 1880 he may have been in WI already.
There are 35 Prochnow's in Wisconsin in 1880. I'd look at each one...then use the links that go to the households all around the ones you find.
And interestingly, there are only 83 Prochnow's in the entire US in 1880...so if you don't find what you're looking for in Wisconsin, look at them all.
...and if you know Charles' father's name, that would help narrow things down a bit.
As you can see, researching this stuff can be quite tedious...and good notekeeping is crucial - which is why I so strongly recommend using software. See
here.
Did you subscribe to the censuses at ancestry.com? If not, then do some hunting in the 1880 and I'll search the 1900-1920 censuses for the Prochnow's, where we might find william living with them.