"kohai".....Here is what others were talking about.
Valterra It's a 3" drain that attaches where your sewer hose attaches (bayonet fitting) and then you attach your sewer hose. This way, if one of your drains leaked, you don't have to take off the cap and deal with the liquid before you attach your hose.
My kids both have travel trailers with the grey water drain in the middle of the trailer and a long handle to pull it open. Even after they dump, water in the middle of the pipe works it's way over to the hose connection while drving and is always a mess. This solved the problem.