Union forces under the command of Gen Samuel Curtis occupied Helena in July of 1862 the city was ringed with several defensive positions. One of these Major Artillery positions was on the bluff above the present day Maple Cemetery overlooking the Mississippi River. That sound like your location.

I believe that General Prentiss relieved General Curtis and that the term "Camp Prentiss" refers to the entire military establishment at Helena instead of an individual encampment or battery position as such. Somebody might correct me on that point.