Shiloh, a Civil War battle site where Confederate forces were thoroughly routed and a decisive turning point in the war, represents the larger forces of history that shape the world the characters of the story inhabit. The characters' inability to understand the significance of the place echoes their inability to understand how events in their personal history--the death of their infant child, for example--shape their current lives.
The Union victory in the Civil War represents the final victory of industrialization and the ushering in of a world in which everything from the bland, familiar subdivisions the characters inhabit to the low-skill, uninspiring jobs they perform, conspires to create people who lack self-awareness, purpose, and thus the ability to create themselves.
Think of Leroy's inability to find a truly satisfying creative hobby. After wrecking his truck rig, he vainly and half-heartedly pursues hobbies like string art and daydreams about one day building a log cabin. It's hardly surprising that he cannot begin to understand why his marriage is crumbling and how to rescue it or begin his life again.
Shiloh was a battle in the Civil War in which there was no victor (the conflict was resolved at another site). Shiloh was also considered one of the bloodiest battles of the civil war. It could allude to the fact that the story has no written or concrete ending.