Water + hand dish detergent. Has worked well, and no smells when dumping or on the road. If it can remove grease from pots and pants, it can do the same to waste matter.
One trick which I've found that helps out is to get a bayonet fit valve and leave it on the end of the slinky hose that goes to the dump valve. This way, you can dump the black tank, close the bayonet valve, then open both the gray and black valves. This will cause the gray water to backflush the black tank. This is a good way to ensure the tank is rinsed without wasting any water.
