I've heard good things about Tank Tech RX which is a probiotic black and gray water treatment but have no experience using it. Has anyone used it? Thanks, Griz14
If you don't want to use just plain water, try The Geo Method, or something similar.
Sometimes, after dumping my tanks, I'll add a cup of lemon dishwasher liquid (the cheap stuff) and a couple gallons of water to each tank. You should always a couple gallons of water after dumping the tanks.
But, plain water works. If you're getting a smell, check under the sinks for an air admittance valve. They go bad.