How do you tell when your waste tanks are full?

In some RVs the pipe to the black tank is not straight so the "look in" method will not work. As mentioned above, once the waste gets above the pipe into the tank it will "burp" when you flush, creating a brief "bubble" in the toilet.

Gray tank water around your feet in the shower is best indicator for the gray tank.

When we are without hook-ups we wash dishes in a dishpan and dump it into the black tank to even out the load on the tanks.