Are we talking solid or liquid manure? Also, is the structure below grade?
In general, if we are talking about a below grade structure, the worst case condition will be when it is empty and only dealing with structural loads on the exterior wall due to the later pressure of the soils (possibly hydrated) and induced loads due to traffic.
If we are talking about a solid manure pile on a above grade structure, the worst case condition would be the manure piled up along the structural wall of the building (note, you may be required to maintain a freeboard (e.g., 0.5') and not stack to the rafters. A 3 (horizontal):1 (vertical) slope is a conservative estimate for any piles of manure (or at least that is what Indiana uses) for determining maximum volume of wastes in an area.