I am absolutely delighted to announce that I have finally fixed my sink drain problem. I can now brush my teeth without having to plunge the sink afterward.
I had a sink vent arrangement under the popup using a hose t and two hoses, but the vent hose actually dipped down to get under the frame mem ber before going up. Water collected in this bend, keeping it from venting the drain properly. I added a short hose under the trailer to ensure that the vent hose only sloped downward; there was no place for a water plug to block it. This works great, and my slow drain problem is gone- the trick is to get a vent in the line as close to where the drain leaves the popup as possible. Make sure the vent hose never dips down, though...
1998 Coleman Cheyenne