Well I finally got tired of an irritating issue with gray water. For several years now we've used a collapsible clear plastic 5 gallon container. It was handy in that, when folded up, it fit in next to the propane tank. This was a handy storage spot. What I got tired of was that after a few trips the container would start leaking at the folds. I was regularly replacing the container. Lately the containers have leaked after only a couple of trips.
We use a seven gallon reliance Aqua Tainer as an extra water container. Harder to find, they also come in a 4 gallon jug.
The large lid comes with a reversible spigot. The 3/4" female pipe threads make this handy. At our local cost-a-lot (true value hardware), in the garden section, I found a pvc connector from female hose thread to male 3/4 pipe thread.
I screwed in the new connector and this is my new gray water tank.
I can securely join it to my drain hose. Open the vent and water easily drains into the jug.
You can buy replacement lids with the spigot. On another lid I replaced the spigot with a plug for transport. This eliminates and chance of spillage or odors. I have done this with our larger 7 gallon water container also.
The empty jug is now the last thing placed in the camper and the first thing removed when setting up. It meets our needs and also, I believe, handles the problem the original poster was addressing - a closed gray water system.