In some places and campgrounds it is indeed illegal to dump GREY water on the ground and if caught you'll get fined.
But that doesn't make any sense since grey water is not really that harmful to the ground because the ground is a natural filter for the water and whatever is in the grey water.
The soil will naturally filter the grey water and send it back to the ground and wells where it can be used again so it should not be illegal to dump grey water on the ground as it's not gonna do any harm to the ground or the environment.
When you flush your toilet it goes to the sewer treatment plant if you're on city sewer and into a septic tank and out onto the ground if you're using a septic tank.
So with a septic tank you're always dumping grey water onto the ground because the excess water leaves the septic tank.