I'm always amazed by the number of RVers who are using gravity to dump. What a slow, messy, stinky mess!
I use a hydro-flush, and it keeps the sanitary hose so clean I actually store mine inside the RV.
The hydroflush is 45 degree elbow cast of some tough, clear plastic. It goes between your normal hose and the RV's connection.
http://www.campingworld.com/shopping/pr ... flush/2096
First off, it's clear, so you can tell what's going on in there and how much more you need to do to get everything out.
Second, there's a jet nozzle on the thing that's you attach to the dump's non-sanitary garden hose.
To use it is simple. You open the dump gate and let the majority of the black water run out.
Then you open the small valve on the hydro flush, and it acts like one of those jet nozzles on your hose at home, and blasts the remaining 'stuff' out. I leave it on for a minute or so, then turn it off, let everything drain, then on for a few seconds, off , on, off.... till the water comes out clean.
Close the black water dump gate, and open the gray one like you would to gravity dump.
Now close the graywater dump, and use the little valve to blow all the smell, yucky, ucky stuff down the drain!.
Unhook the garden hose, open the little valve to let air in, and the clear water drains out of the sanitary hose.
Unhook - your sanitary hose is clean and non stinky!
It's $20 or so, and far and away the best gadget I ever bought for an RV. I'm on my 3rd one only because I felt obligated to give the new owner mine when I sold my old RV's.
It's sturdy and well made. Strangely, it's got a pretty cheap valve attached to it. I replaced one valve with a nice bronze one, and would do it again if I could find another of those bronze valves.