Lots of good advice and a proper maintained drain and waste system is essential for a good camping experience.
The reason I brought up the "closed" valve when filling from this flush connection was because some have talked about the anti-syphon leaking when using this connection. I have read where others said they removed it from the water line....
If a person needs to be told that leaving it closed when filling will overflow the toilet, perhaps they need to be told not to go back to their trailer to make a cup of coffee while still driving down the highway
These anti-syphon valves are CHEAP, extremely cheap, from one who has spent his whole working life as a plumber and dealing with backflow prevention.
A kink in the pex going to the black tank, or an obstruction in the line at the black tank, even a small wad of toilet paper, or a kernel of corn off that cob wedged just so....can cause back pressure and make the valve "spill over"
It's doing as designed, just poorly.
Why I mentioned the tank valve being closed, it presents back pressure too, as the tank fills, like blowing up a balloon, the more air you blow into the balloon, the harder it gets to expand....
Also, most trailers I believe have "mechanical vents" not open to the atmosphere like our house, they work by "emitting" air into the system to allow for drainage without sucking the traps dry under the fixture. They use these because our vent terminals are so close to our open windows, camping area outside, where the fumes could easily encompass us with their stench....
So they "close" too when back pressure is exerted on it, like filling the black tank quickly with the flushing hose.
Moral of the story....if your flushing system leaks when using, try leaving the valve open, using less pressure in the water hose feeding it, and see if that helps.