[Yamahafx4]

Originally Posted by sourdough

Just throwing things out but first, what year is your trailer; you don't specify anywhere.

If you are running water into the black tank flush, is water then coming out your drain hose? If so the anti siphon valve is correct (and you would probably be soaked in water either inside or outside if it wasn't).

Once the full black tank has drained it is a straight shot to the sewer system it's connected to; do you have anything drawing those odors in like the vent Javi mentioned? The toilet flange seal could be worn, askew or otherwise allowing the odors in. Likewise the ball seal in the toilet but if it's holding water you could rule that out.

Back to the anti siphon valve; to eliminate this and other probable issues that will come from it you could replace it, or better yet, eliminate it to rid yourself of those future worries. Just remove the valve and place a 90 degree fitting in its place. When done put a screw on anti siphon valve on the end of your flush hose and you're done.

Sorry

2020 Avalanche 376RD