My black tank and gray tanks have more capacity than my fresh water tank. So when my fresh water tank is empty I go to the dump and dump the waste tanks and fill the fresh water tank. If you only use water from the fresh water tank it is unlikely you would ever overfill the black tank.
The secret as has been already let out is lots of water with the black tank. So not peeing into the black tank actually helped create a problem that sometimes is very difficult to fix.
After I do my #2 business with the toilet I fill the bowl full with water before I flush. Never have had a problem and most of my trailer use is dry camping. Two of us have gone 5 days before we run out of fresh water.
