According to Stanton, Order of Battle: The 115th FA Bn was equipped with truck drawn 105mm howitzers. Organized 7 Feb. 1942 at Ft. Jackson, SC from the 1st Bn., 115th FA. Boston Port of Embarkation: 21 Aug. 1942 - (Relieved from the 30th Inf. Div. 24 Aug. '42) arrived, Iceland 31 Aug. 1942. Arrived England, 6 Nov. 1943 and landed in France 23 September 1944. It took part in four campaigns: Northern France, Alsace-Lorraine (Battle of the Bulge), Rhineland, Central Europe. It was stationed at Weiler, Germany to 20 April, 1946.
Try contacting the National Archives and Records Administration (NARA II) at College Park, MD
The National Archives at College Park, Maryland and The US Army Heritage and Education Center, formally the US Army Military History Institute at Carlisle, PA
The U.S. Army Heritage and Education Center (USAHEC) where I am sure you will be led to a unit history.
A little advice: take family history stories with a grain of salt. Family members mean well, and harbor the best intentions, but tend to relay facts very broardly. When I would ask where my father fought in WWII, I would get answers like "he was everywhere!" And ... "your father fought against Rommel!" It took research to discover that my father landed at UTAH beach on D+28 while Rommel was commander of Army Group B...so...yes, my father fought against Rommel, which would be the same as a German corporal fighting against Eisenhower. I hope this helps.
Greg C.