I personally don't have a problem dumping some or all of my grey tank onto the ground. That water is cleaner than if I washed my RV and the residual water and soap run off. Now, what would stop me is how is if I have a neighbor on that side of my campsite and how close is the neighbor.
I know some folks use the adapter cap that you put on the sewer outlet that lets you attach a small hose too. They'll then crack open their grey valve to trickle drain the tank as it's being used. I actually bought one of those and kept a small old washing machine hose in my rig, but I never used it so it is on a shelf with other RV supplies in the garage.