One other consideration. Sometimes if you have not used the shower drain for awhile, the trap may dry out. The water evaporates after time and this can be accelerated if you are on a rough road and slosh some of the trap water down into the gray water tank. In such instances, less water in the trap will evaporate quicker. In any case, first thing to do when you get a bad smell coming out of any drain is make sure the trap is full of water. Just pour some in there.
If the water in the trap is reasonably clean, you should not have any smells coming up out of the tank because they are sealed out by the trap.
Also, if you trap is dry when you run a roof fan you will pull air out of the tank up through the trap. You will really have a strong odor then.
If your trap does have water in it and it still smells then your problem may be gunk stuck in your trap. A forceful rinse of the p trap followed by a little bleach should clean your trap.
If your trap is clean and full of water you should not be able to smell your tank.
But yes, keeping your tank clean with some bleach and well flushed like Gary and Mike say will help a lot – especially when you empty it.
One other tip – check your plumbing roof vent openings for wasp nests, etc. If your roof vent is clogged then when the tank is drained it will pull water out of the trap.