On some Thetford toilets there is an overflow tube that extends from near where flush water enters the top of the bowl directly to the black water tank. This is apparently intended to keep the toilet from overflowing if there is a small leak in the water valve.
The tube is curved, forming an "S" trap. If there is no water in the "S", there is a direct passage between the black water tank and the bathroom. Thus - BAD odors in your living quarters - regardless of what chemicals are in the tank.
To fill the "S" with water, simply fill the toilet to the brim. Let it stand for a couple of minutes to make sure the overflow trap is filled, and flush it down. Then, just use the toilet normally. It would probably be a good idea to repeat this a few times during the camping season, just in case the trapped water is sloshed out or evaporates.
For a diagram showing the overflow tube, see:
http://www.thetford....18/Default.aspx
