Originally Posted by Burnhaven
Just bought this 1999 30RL recently and didn't realize there was a third drain handle underneath. So it looks like the two handles nearest the exit pipe are for the two gray-water tanks, and the handle farther back under the trailer is for the black tank.
Yep. That's what I see.
If you are hooked up to sewer it is OK to leave the gray valves open. But not the black. The black water tank has to fill with solids and liquids at the same time. If you were to leave the black open you would end up with a big pile right under the toilet.
When dumping you will want some water in your grey holding tanks to help rinse the hose. If you have a San-T-Flush that is good. If not get a wand or at least stick a hose down the toilet to help clean the tank.