1.  Jordan R. Dodd, ed.  1998.  Marriage Index: Missouri, 1851-1900.  Liahona Research, Orem, UT (Broderbund CD-234):

Davis, Joseph	Hardin, Martha	6 Apr 1855	Boone Co.
Davis, Joseph	Hardin, Elizabeth	23 May 1859	Boone Co.

2a.  1860 Census Index/Images (online at Ancestry.com):  can't find Joseph & Elizabeth, but they would be newlyweds.  They left Isaac's daughter with her grandmother.

2b.  1860 Census Index/Images (online at Ancestry.com, Image #10 of 44):  Columbia P.O., Rockyfork Twp., Boone Co., MO, Roll M653_608, p. 646, PN 10, 67/67, enumerated 7 Jun 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):

Nancy Hardim/Hardins	60	F			2500	1000	Ky
Nathan      "	32	M		farmer			"
Sanford     "	17	M					Mo
Arretta Davis	4	F					"

Nancy is Isaac's mother-in-law.

3.  1870 Census Every-Name-Index/Images (online at Ancestry.com, Image #78 of 83):  Sturgeon P.O., Township No. Fifty, Boone Co., MO, Roll M593_760, p. 232B, PN 78, 1/1, enumerated 24 Aug 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):

Davis Joseph	36	M	W	Farmer	2500	350	Mo
_____ Elizabeth	26	F	W	Keeps House			Do
_____ Aretta	14	F	W				Do
_____ Fannie	10	F	W				Do
_____ Geo	6	M	W				Do
_____ Daniel B	4	M	W				Do
Hardin Nancy	66	F	W				Ky

This enumerator had 1/1 for everyone.

4.  LDS.  Family Search: Census Records: 1880 United States (online at FamilySearch.org):

Census Place: Rocky Fork, Boone, Missouri
Source: FHL Film #1254676; NARA Film #T9-0676; Pg 244D
	Relation	Sex	Marr	Race	Age	Birthplace	Occupation	Fa	Mo
Joseph DAVIS	Self	M	M	W	48	MO	Farmer	KY	MO
Elisabeth DAVIS	Wife	F	M	W	39	MO	Keeps House	KY	KY
George DAVIS	Son	M	S	W	17	MO	Farm Labor	--	--
Nannie DAVIS	Dau	F	S	W	9M	MO		--	--
Nancy HARDIN	Mother	F	W	W	77	KY		VA	VA

5.  1890 Census:  the 1890 Census Population Schedules were destroyed.

6.  William F. Switzler.  1882.  History of Boone County, Missouri.  Western Historical Co., St. Louis, MO.  On p. 1107, under the heading, "EARLY HISTORY":

Isaac DAVIS came from Christian county, Kentucky, to this part of Missouri in 1818, and settled two miles east of Hallsville.  His sons were James, Henry, Isaac, Benjamin, John and Joseph.  One of Isaac's sons, Joseph, now lives on the old DAVIS homestead.

The Joseph here is son of Isaac son of Isaac, that is, grandson of the Isaac who came to MO in 1818 — in other words, our subject.

7.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).