I have a dump for grey only. Thankfully a few dollars from our RV tabs goes to fund dump stations at nearly all of our rest stops. There's also Camping World. They don't mind if you pull behind the shop and use their dump station. Generally there's no one there!
I have a 4" pvc cleanout in my driveway (regrettably the wrong side of driveway but still close enough I wont complain) I'm not sure of a reason not to dump there. goes directly into the sewer. Black first grey to wash it down.