We ran into a problem in California, which has a law that all sewer connections must be airtight. One campground we stayed at wouldn't allow us to run our greywater hose into the sewer
. (But they did allow us to drain our greywater into an open bucket
I pondered on how to be legal, and of course, look good too!
I found the first part below at Camping World. It's supposed to go into the sewer drain at the end of the drain hose, with a rubber washer thing to seal it into the sewer drain. But when I connected the hose to it the drain plugged up due to air locks in the hose. Then I realized that the grey part will screw directly onto the sink drain on the trailer. So I bought the red universal sewer connection and a length of slinky hose and voila! There's no way this puppy will plug with an air lock.
Photos below taken at home, not in the "real world"