I thought the physics of the matter was the core of the question. Obviously they just leave waste in the area the ship was positioned prior to movement.
The tricky situation is the mechanics of ejecting waste. I'm going to lay a bunch of educated guesses:
Mess and Containment
I don't think that they can reliably eject raw organic waste into space. The surface tension of the water in the waste is likely to cause it to stick to the hull of the ship after 'ejection'/ This would lead to that part of the ship looking like the butt of someone who never wipes after pooping.
The trash depicted in 'episode V' is containers or compressed waste. We can suppose that organic waste could be entombed within such a container and abandoned. I don't think this is the correct line of thinking.
Organic waste would be immensely valuable for any space craft, isolated from an ecological environment. Systems for breakdown of organic waste and excrement are known today, and there is no reason to consider that composting would be absent, as a process – particularly when we know that it can produce energy even with the tech we have today. Much of the organic waste stream could then be tempered (see above) and used for food systems.
So I presume that any of the Star Destroyer class vessel, and above, would be sufficiently sized to make waste reclamation a worthwhile operation. In this situation, most organic waste would be reused. Anything particularly problematic to that process could be separated, dried, compressed to bricks, and ejected.
All of this presumes that the resupply periods are sufficiently long that increasing sustainability of the ship is worthwhile. This seems inevitable, but I welcome anyone deeply-read to correct me on that.