When the blackwater is full

Yes...

I would think that if it is REALLY full you will see liquid coming up the pipe connected to the toilet and the black tank.

I know the grey water is full when it comes up in the tub. This was on our first outing and didn't realize how quick the tank would fill up.
A lesson well learned.

Yes but I wouldn't get it so full that it back up into the neck of the toilet.

Yes. My tank is cracked on top somewhere so I don't let it get that high. If you do let it get that high, you're really testing the limits of the seal between the tank and the toilet. I wouldn't recommend that.

The drop pipe Some are slip fittings, some are glued

I don't want to see mine that full, it might leak into the floor or the basement

If your gauges aren't working then use a flashlight and check it often

MrWizard wrote:

If your gauges aren't working then use a flashlight and check it often

They work, but I don't get much "resolution". 4 LED lights that indicate full are not that helpful when trying to determine if the tank is full - full.

yes but if you can see it it is probably over full.

It will likely start “burping” before it gets that full.

pianotuna wrote:
yes but if you can see it it is probably over full.

I think you are on to my real question. I've been referring to "full-full" and I now know what I will be able to see when it is "scarey-full".