I looked at a TT (ran away, once I found out what happened) where the seller did exactly that. He actually set up a relay system to his truck where he could open the gate valves on his TT while driving.
The problem is that he let loose right in front of a Texas DPS officer (DPS is the same thing in Texas as the CHP is in California.)
Result: $3500 fine (I think jail time, but the seller wouldn't fess up to it.) He then just dropped the black tank and let the "output" fall under the rig. I noticed this when I took a flashlight to the toilet, saw driveway underneath.