Here are a couple of hints:

HEADQUARTERS FOURTH ARMY CORPS, No. 12. Nashville, Tenn., April 29, 1865.
I. The encampment of the Fourth Corps near Nashville will be called Camp Harker.
War of the Rebellion: Serial 104 Page 05134 CORRESPONDENCE, ETC. - UNION

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See also : Civil War Defenses of Nashville
https://www.northamericanforts.com/East/tn-nash.html#nashville

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April 30th
I am feeling some better this morning. Still laying in camp. The name of this camp is Camp Harker.

May 1st 1865
The 45th O.V.I. is laying in camp five miles from Nashville.

May 6th
I am in Nashville, to see the city.

https://ohio45.homestead.com/mus42.html