I just spent a week at a full hook up site. I had to use the 'magic wand':
each time I flushed the black tank. After way more time than I had to spend with that wand, it broke loose a ton of "SOMEONE'S" four tons of toilet paper. Finally got it going, but it was a bugger bear.
Since it finally broke everything loose, I'm convinced my plumbing and pipes and valves, etc., are all correct.......just too much 'crud' in black tank. And yes, it was full of water before I flushed it. It would start, but never finish. I apparently am one of the luckier ones that my tank sensors seem to work perfectly. And only after repeated use of the wand did it all break loose. And.......this was a NEW black tank......my first use after my 'black tank incident' late last fall. So it wasn't full from before.
I really do believe that from now on I'm going to try and travel with SOME water in the black tank, and let it 'slush around' in transit. I have a 5 week, 3700 mile trip, starting Tuesday.........so after first stop at FHU site, that's what I plan on doing.
Dunno if that's your issue or not, just a thought.