hi can you feel any resistance when you're pulling on the cable does it feel like the other ones when you pull it out you can actually feel the gate open.
The cable might've come in the attached from the gate valve you might have to open up underneath your camper and find a valve and have a closer look .

Yes the cable does come out quite hard. I am going to check the cable tie in from underneath to see what's happening. Will reply. Thanks

Does your drain have the HepvO Valve? Mine does and I had a hell of a time getting it unplugged.
I tried the hot water and vinegar/baking soda method, then my Zip-It tool which probably wasn’t good for the valve. Finally I used a length of Cut-N-Tie which is a flexible rubber coated wire that you cut to length and use as a tie wrap. I got that down far enough moving it all around and eventually it cleared.
Good luck!

Don't have HepvO valve.
Checked cable connection under rv. Found it to be working . Now cable handle works fine outside but not draining . Reversed flush from sewer termination outside but to no avail. Maybe not enough pressure at this rv park in Michigan.
Probably going to try bio active tank treatment next.
Otherwise I'm clueless.

ok but maybe the tank is empty ?
Up the valve and have someone run some water to see if anything flow out.my stupid lights all ways show half lol.

not sure maybe draino or plumber snake

no draino.

Hi Dave, we had this issue a couple of times. Have you changed elevation significantly right before this happened? Just like a bottle of water will collapse in on itself and form a suction when changing elevation, the tanks will do the same thing. After making sure there was no issue with the air intake for the gray tank and that the shower and sink drains weren't closed, my husband used the softer side of a rubber mallet to tap (okay whack) on the drain line, on the "tank side" of the gate valve. After a few hits, WHOOSH the suction broke and it drained. This has happened to us twice, both when we had 3000+ ft changes in elevation between the last time we dumped and this time, and only on the gray tank. Good luck!

You may want to check for a blocked vent. All the holding tanks should be vented to the outside and shouldn't have a pressure differential like a closed bottle of water would.