>>What was the longest serving US navy ship of the past century?<<
That would likely be some of the old tenders though the names escape me at the moment. Currently, the oldest vessel in active service, short of the USS Constitution, would be the aircraft carrier USS Kittty Hawk. She was commissioned on 29 April, 1961. She's scheduled to decommission next year after 47 years of service.
>>The USS Wisconnsin had quite a long life...Wasn't she a vessel from WW2 that also fought in the 1st Gulf War?<<
The USS Wisconsin (BB-64) is an Iowa class battleship and is currently a museum vessel moored at the Nauticus Museum in Norfolk Va. See Battleship Photo Index BB-64 USS WISCONSIN for a photo study of the ship which served in World War Two, Korea, and Gulf War One.