After dumping, when you open the black valve and put on the sprayer, do you immediately put on the pump? Or do you wait a short while for the bottom of the tank to fill and then put on the pump for a few minutes? After dumping, how much water should be added to the bottom of the black tank- a few flushes?
After starting to spray I wait 2 or 3 minutes before starting the pump. The spray cleans the walls and the top, flushing cleans the bottom.If the pump runs out of water, I stop it for a couple of minutes more. I continue doing this until the water coming out is clean. After dumping the black tank, you should fill the toilet bowl and flush it so you have some water in the bottom.
Never put the hose into the sewer hole and leave the black tank open. What happens is what Ron had happen to him, big stink every time you flush the toilet.
The gas in the sewer will come right into your bathroom. The other problem is without water in the tank, you will build up a cone of waste that will get very hard and eventually clog your toilet. Not a job you want to have to clean out.
You can leave the gray water open because the shower and the sinks have a "P" trap under them that is full of water, keeping the gas out of your bathroom. I always run water into the sinks before each trip to fill the traps, after that normal use will keep them full.
The gray water bypass I was talking about is on the sani-con pump, not the 3 inch pipe facing out the back of the motorhome. That is the other way to dump, if you have a problem with the sani-con system. RVs with out the sani-con have this type of hookup to dump with.