I'm sure that all of us have removed the twist off cap to the main sewer outlet, only to find ourselves leaping out of the way to avoid the "stuff" pouring out before you can get your sewer hose connected.
I am finding that the 3 pull rods used to release the contents of the 3 waste holding tanks on our Cougar are a bit on the "flimsy side". I have been left holding just the handle on one occasion and the handle and rod on another.
I am never quite sure if the valves are completely shut when I push them back in. And the whole system is quite inaccessible since it is almost entirely enclosed in the underbelly.
To help reduce the risk of these sudden and most unwelcome outbursts of stuff from the sewer lines when removing the cap, I have purchased and installed a twist-on waste valve made by Valterra. It simply twists on your existing sewer outlet (just like the cap) but has a sliding pull-out valve which, when closed, will prevent any sewage (grey or black) that may have leaked out into the main line from spilling out when the cap is removed. It also serves as another valve at the end of the line in case you forget to close off one or more of the 3 tank valves. But who would ever forget to do that?
I had to cut off about 6 inches of the main sewer pipe so that it didn't protrude outward beyond the side of the RV. Had to buy another fitting and glue it to the main pipe.
You can pick up one of these Twist On Waste Valves from CW or most RV places for about $25-$28.