I have the next model up, the 208. Ours had a kink in the line under the folding sink as it was pinched between the cabinet when setup one time and that constricted the flow. Also, the regular drain line didn't have a vent, and then necked down to the 3/4 hose as it exited the camper. the 3/4 was fine if it was a short hose, but more than 10 feet and we could really wait for the water to go down the drain. Sounds like that is what you have. By jiggling the line, you are letting air bubbles up the line and helping relieve some of the vacuum.
I replaced that with a PVC adapter and added a vent, then went from there with a piece of sump pump hose which is larger and lets things flow smoothly. Works great now.
Here's a link to a few years ago when I fixed my drain line and connections. You can see there what I did as well as photos of my condition to see if it is like yours.
Sink Drain Fix -->
Outside Drain Fix / Grey Tank -->