Wand
I presume you're camping with black tank closed, and flushing at 2/3 or so. 1) use a clear elbow at the end of the hose to see what's going down. It can happen that the volume moves enough solids to plug the hose, but this usually flows after a bit, although you may think it's finished. Shake the hose. 2) get a "black tank sensor wand" and bring it in a window attached to park water. Step on the toilet handle and flush up and down and twist it. Mimics what they factory install to clean off the sensors. Add fresh water and flush again. Should run clear. Repeat till you get tired or sensors read zero!
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