The question has been asked, when hooked to a site with sewer, should you keep your valves open or closed? The answer is closed. Some people think keeping the sewer valve open is best so the tank doesn’t fill up on extended stays. The opposite is true. You need lots of liquid to flush the solids. You want your black water tank to have a lot of liquid in it (about 2/3 full) before you open the valve to dump so that:
1. Liquid will help to break down the solids and toilet tissue. Speaking of toilet tissue, be sure to use RV toilet tissue. It is specifically made to dissolve quicker in liquid. The last thing you want is a pile of undissolved absorbent tissue, drying into a solid obstruction at the bottom of your tank
2. You want the pressure from the liquid to sweep the solids with it and flush the tank clean. In Fact, it is helpful if you run even more water through after you open the valve while emptying it. Always empty the black water tank first. After you close the black water valve, you should have plenty of gray water to flush out the hose. I usually open the kitchen and bathroom faucets to run a few gallons more through the tank before I open the gray water valve.
Keep the valves closed when connected to the sewer, so you don’t get the sewer smell coming back into the unit. You also don’t want to open the door to things that live and breed in sewers. There can be roaches or rats living in the sewer. Most commonly, you may get Psychodidae, often called drain flies, sink flies, filter flies, sewer flies, or sewer gnats is a family of true flies. Drain flies sometimes inhabit plumbing drains and sewage systems feeding on bacteria. They are harmless, but may be a persistent annoyance.
How do you get rid of drain or sewer flies?
If you find yourself with a drain fly problem you can get rid of them on your own.
Pour in 1/2 cup of salt and 1/2 cup of baking soda plus a cup of white vinegar. Allow it to work its magic overnight then flush the drain with hot or boiling water the next morning. This will sanitize the drain and kill the flies and their eggs.