"we can find no death certificate and no burial information in either WI or PA"
Wisconsin wasn't a state until 1848. Its vital records are inconsistent until 1907. The earliest death records filed in Dane County are from 1876:http://www.dhs.wisconsin.gov/vitalrecords/genereq.htm#G5
"we only know they lived in Dane County no idea of which town or farm they lived"
Where did David's brother, Peter, live? That would be a good place to start looking for information.
". The other children were divided amongst five different families."
Where did those families live? Those would also be places to look for information.
David may have bought and sold land in Wisconsin. Dane County property records and indexes have been microfilmed by the Family History Library. You could look at those.
Some Dane County tombstone photos can be found here:http://www.usgwarchives.org/wi/cemetery/dane.html
Other cemetery indexes can be found in the newsletter of the Wisconsin State Genealogical Society.