If your cleaning and dumping your tanks correctly, there shouldn't be any buildup of sludge in the bottom of the tank for them to get stuck in.
If you don't get them from the top, you can always try filling the tank all the way to the sel of the potty and pulling the valve. You may want to consider that if your doing this at home, you don't want them to get into your home's piping system, so you may want to let the big flush, I mean rush of water go out onto the lawn. It won't hurt anything. And the grass will probably be greener on that end of the hose the next day.
My blackwater story is titled "Always use your own dump hose". I used someone elses one time, and about 1/3 through the black water draining process, the hose came off the connector. It's amazing how fast a good set of tinny's can fill up, AND the looks you can get as you walk into a store and put a foot into the restroom sink to wash certain marks off. It also amazing how well I can recall that event every time I pull the handle. It always makes me hesitate, then step off to the side abit.