This is the first issue we ran into after purchasing our first camper. We tried everything and nothing worked. A friend of mine recommended the "Tornado". Now some campers, especially a lot of the upper line models may have this already installed. On our Jayco Designer they call it a black water flush. It kind of works like the "Tornado". You have to do a little manual labor to install them, but it's not bad. What it does is fits into the top 1/2 of your black water tank and sprays a high pressure stream inside the tank, losening up paper and buildup, which then filds it's way then to the sewage and out the drain. If you aren't comfortable installing it, most trailer sales or places like Camping World will install it.
When you are ready to dump, you hook up the water hose directly to the remote hose attached to the "Tornado". You let water run through it a few minutes. The first time you use it, it may take a while before you see clear water coming out of the dump drain.
I also make sure that I keep black water chemicals in the tank with a gallon or two of water if the camper is going to sit for awhile between camping trips. I also recommend installing at least one vent cover so you can leave the vent open.
Another way to clean out the black water tank is to back flush from time to time. What happens sometimes is if you dump and don't fill up your tank, there may not be enough water pressure to force all the chemicals and "waste" out. What happens is this buildup starts to hardened into a sludge and produces that bad smell. You can find this type attachment from Camping World here: http://www.campingworld.com/browse/skus/index.cfm?skunum=20522&src=SRQB
This is the one we have. It's called a Flush King and works like a champ. These are some of the ways we have solved the problem, but I know there are many others. :thumbup2: