My experience with this is a bit different than what people have posted. I have not been cited but, I have friends that have been cited and I have watched people in my camping areas be cited. I cant say if its, county, state or federal. I have spoken with a number of BLM rangers over the years and the answer has always been the same, if we see you dumping or we see where you dumped (hose on the ground and it looks like water flowed) we will ticket you. I have been told the same applies to national forest and national parks but have never talked to the rangers there.
The vehicles I have seen targeted are motor homes and campers. My M-100 and RTT have never been in "common" camping areas where they inspect.
What I did with my camper was, find a camping spot near a large bush. Drain into the center and put the hose away.
I can see why its illegal. I've watched people dump black water onto the ground and have the argument of its biodegradable.
I don't want this to turn into a rant. I'm just sharing my experience.