Black Tank Filled?

I concede that it is possible to fill the black tank first but as you outlined, one would really have to work at it, replenishing the potable tank. No amount of outside showers or dumping gray water on the ground is going to cause the black tank to fill first, or at all, unless the potable tank is replenished, in which case one would hope the camper would be aware of the potential black and gray tank issues.

And yes, it's possible to fill the black tank so full it will overflow when moving.  That can be done with any monitoring system if no one monitors the system or it doesn't work.  Any method of monitoring that works and is used can prevent that though.  Of the three monitoring systems being discussed, sensors, sight glass, looking down the toilet, I maintain that the one that will work most reliably is looking down the toilet.

TT

So yeah, if I did it wrong, (as if!) I could end up with a full black tank and an empty or nearly empty grey tank. As Tar says, I'd have to really work at it. Other campers with no blueboy might dispose of grey water by local draining, OR, produce very little grey water, but the black tank is not being emptied. 

Some boondockers with a failed fresh water pump (or leaky plumbing) have used portable jugs to pour water into the commode during use. You know...'stuff' happens.

But in the general RVing public, things have been known to be done incorrectly...all one has to do is watch youtube to see people doing dumb things and then monetizing the video....so maybe it's not THAT dumb if they cry all the way to the bank...