Undoc'd source puts death at 1889.
The Undoc'd source has been pretty accurate.

On DOB...If she was 21 when she got married on 7 AUG 1888 and she was still 21 when she died 3 MAY 1889, that means she was born BTW 4 MAY 1867 and 7 AUG 1867.

in the Iowa, Marriage Records, 1880-1940
Name: Tillie A Wagner
Gender: Female
Race: White
Age: 21
Birth Year: abt 1867
Birth Place: Blackhawk Co. Iowa
Marriage Date: 7 Aug 1888
Marriage Place: Buchanan, Iowa, USA
Father: Wm. Wagner
Mother: Emelia Tribon
Spouse: Robert T Crawford

in the Iowa, Cemetery Records, 1662-1999
Name: Tillie Crawford
Birth Date: 1868
Death Date: 3 May 1889
Age: 21
Burial Location: Washington, Buchanan
Cemetery: Oakwood
General Burial Info: (add. 1, lot 9) (Robert T. and Sarah A. buried on this lot)
Source: Buchanan County Burial Records
Page Number: 98

So, Tillie died ~9 months after marriage suggesting, at least, it could have been death from child birth.

I have been unable to find any additional marriages for Robert before his death in 1901.

About the headstones...(Courtesy of Hiesela)
Crawford Family Plot
Stones from left to right:
Robert - Wife - Sister
The large stone is engraved on the front:
Robert T
1st Lieut 1st Inf USA
Sept 5, 1863 - Oct 30, 1901
Drowned at Samar while striving to save his men.

The Regiment Commander extends congratulations
on your brave defense and successful repulse of the
insurgent attack on Hermosa the 25th & 26th Insts
(Maj. Spence to Lieut. Crawford Sept 29, 1900)
CRAWFORD

The right side of the large stone is engraved:
Wife
Tillie
Died May 3, 1889
Aged 21 years

The left side of the large stone is engraved:
Sister
Sarah Alice
DCE 12, 1871
DCE 25, 1898
Added by: Hiesela on 19 May 2008