If you take out the drawers and look to aft side of the furnace you will see the drain line vent. On top is a screwed in adaptor as shown in the attached picture. [img][/img]
Remove the adaptor and the sink will drain much better. The adaptor is a vent and also prevents water from coming out of that vent I think if you remove the adaptor the possibility of water backing up that high is very low. If you make a small extension so that the vent opening is above the bottom of the sink, then the possibility of a water problem is very low. The shower would also back up before the vent will.
The adaptor also keeps sewer gases from the grey water system from coming in the trailer but I keep the tanks clean and do not leave the drains open when hooked up, so odor is not a problem.
I took out my vent widget when I added the folding sink shown here http://escapeforum.org/index.php?topic=1550.0
and the galley sink flows better and I have not had any backflow or overflow problems.