I am confused on why I can not locate deeds in Coos County, New Hampshire for the town of Milan in 1790?
The reason you are not finding deeds that early is due to the incorporation of the county. Coos County was formed in 1803 from Grafton County, New Hampshire. The deeds you need would be under Grafton County. We have these records on microfilm at NEHGS on the 4th Floor Microtext Library (Call # F42/G7/G47).