Is the sewer inlet higher than your tank drain, so nothing flows when you open the valve, or is it draining, and you're just concerned about leaving a hose full of nasty stuff on the ground?
If the tanks are draining, just delay dumping the black tank until you have a close to full grey water tank. Dump the black water first, then close the black tank and open the grey valve.
The grey water will flush any black residue in the hose down the drain, and all you'll have is a hose full of grey water.
When you're ready to leave, and have a hose full of grey water, is the time to walk the hose.