Many, many, many years ago when my Dad was feeding livestock on a small scale for a couple of years, he was getting about a ton of feed custom ground and mixed at the small local feed mill.

We generally furnished either burlap or paper baga for the feed to be sacked in.

They had a way of filling the bags that worked well.

The bag filling location, depending on the feed mill, had  one to two spots for filling the bags. 

He, the bagger, would pick up the bag, hold the open end under the outlet, slide part of the bag over the outside of the outlet, then grab the handle of the bag holder clamp and clamp the bag in place.

Then he pulled the sliding gate out of the outlet enough to begin filling the bag. When the bag was filled, he pushed the sliding gate in to shut off the flow of material.

Then the bag was unclamped from the filler outlet. The open end of the bag was either stitched or tied shut.

And the process was repeated until all of the entire mix batch was bagged and loaded into the truck or trailer.

With regards to filling the sandbags, you will need a big overhead storage hopper with the same lower manual bag filling equipment that was used in filling small custom mixed feed batchs.

Sorry, I do not have any pictures or drawings of the operation from long a ago.

I think that a search through the US Patent Office archives of the many and varied patents should yield the patents and drawings of such bag filling processes and machinery. It will be a learing lesson into the filing system and etc that the US Patent Office uses.