First, ensure you always have water in the bowl. This will prevent the entry of odors into the space. Second, keep the drain valve closed until you are ready to dump the tank. Third, always use plenty of water when you flush.
Get a clear extension for your sewer line, then when you dump the tank for the final time make sure you flush it out. Get a wand and stick it down into the tank. Move it around to get all the areas you can rinsed off. You are not supposed to close the black tank valve when flushing it with the sprayer, but I do anyway in order to build up lots of water. The more water you use, the cleaner your tank will be. Finally, when the black tank is running clear (as seen through the clear extension) dump your grey tank. You might want to rinse it out as well if it doesn't run clear. Residual scraps, coffee, and such can rot in there, making for unpleasant conditions.
Finally, after all is flushed out and nice and clean, add a gallon or two of water (enough to cover the bottom of the tank) and your favorite chemical. I use some kind of citrus stuff I bought at WalMart that goes a long way.
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