I had the same problem a couple of years ago and anticipate I will have it again this year.
I went to an RV Repair place and talked with them about it. I was told to take a rubber mallet and tap the tabs on the clear and it should break loose.
I tried it and it worked...but make sure you lightly tapand just keep going from tab to tab. I evidently tapped too hard as I broke one of the tabs off the main sewer dump and cracked another.
It turned out not to be a big deal with the broken tabs, just cut off the good tabs and get a new termination fitting and glue it on over the old one.
Hope someone comes up with a better idea as I will be moving in a few weeks.
Paul...