We use a gray tank deodorizer/cleaner product I think is sold under the TST brand. It's a lemony liquid and you can get it at Wally World. When we put Daisy away for awhile, we pour a little down each of the drains followed by a gallon of water equally distributed. This sits in the tank while we're not using it and when we are back on the road, we dispose of this tank in an RV dump station. We have never had any problems or smells with our gray tank following this procedure.
When we are traveling (moving each day instead of base camping), our daily routine is to use the pump and water in our fresh tank. Each morning we dump the grey water and refill the fresh tank. This keeps both tanks fresh and clean and we know the pump is working too.
If we are base camping and connected to a sewer, we let the gray water drain until the day before we need to dump the black tank. We save some wash water to use as a rinse for the hose. The only time we add any chemicals again is when we go back into storage.
The "old timers" have told me that when you dump gray water on the ground, where its allowed
, you should dump it as it is made to prevent the possible side effects of too much wash water. To be as green as possible with a ground drain: (1) Strain all your wash water to make sure no garbage goes down the drain (you can get a sink strainer at Bed Bath and Beyond); (2) Drain at least 200 ft away from any natural ground water (rivers, streams, lakes, etc.), and (3) Eliminate all antibacterial soaps in your trailer--use only natural soaps. Following these rules you would do no harm.