Sweet rig!
Your said boondocking, so I'll assume dispersed, no camp sites, or no facilities like a lot of NF campgrounds.
The enviros will poop a biscuit when they read this, but black tank should last a long time. Gray tank shouldn't overflow with available water from your water tank. Depends how they're sized I guess.
But if you refill with water and fill the gray tank, if the topography and neighbors locations permit, dump the gray tank on the ground. It's just soap water. If you want to feel good about yourself use some earth friendly soap.
I bring a gray water garden hose just for this on boonie trips. I refill with water once or twice and dump the gray as needed.
That said, don't gross out your neighbors but 50 gal of water in the qualities behind camp don't hurt nothin.