Our coach has a black tank flush, where it can rinse out the toilet tank, but doesn’t have a similar mechanism for the gray tank. It generally gets cleaned out by filling it up with soapy water from showers, laundry, and sink usage, but occasionally an extra rinse is beneficial.
To do that, one could just close the gray dump valve and let it fill up with regular usage, or one could fill it via the dump valve.
To do the latter, I have a Camco Dual Flush Pro Holding Tank Rinser, which connects in the sewer pipe and includes a gate valve and hose connection, and a knob to direct the water towards the tank or towards the outlet:
By closing the gate valve, opening the gray valve in the wet bay, and turning on the water, it backs up into the gray tank.
To avoid over-filling the tank, I watch the level indicator in the wet bay, and also a water flow meter connected to the hose:
Here’s the whole setup, including the wet bay:
For best results, this can be combined with Liquified Tank & Sensor Cleaner, to give a deep clean of the tank.