For those of us that like to automate things...the Sonoff / Barker black tank valve project thread and this one comes together.
T off the fresh water line to an electric water solenoid (electrically connected to another Sonoff relay or a ZWave valve) to a T on the Black tank (and / or Grey) flush lines and have an automation controller sequence and time the flush.
https://www.amazon.com/Motorized-Val...ag=googhydr-20
I would put an electric switch in the water bay that is manually switched when the waste system is connected to a campground septic system.
Then, with each programmed tank flush, automatically fresh water flush the tank. Close the fresh valve, close the waste valve.
For worriers, put a manual shutoff valve between the fresh T and the solenoid to put the system into Manual mode when you disconnect the RV from the campgrounds facilities. Follow that by switching OFF the electric switch.
As a further safety, one could also wire into the black tank level sensors to never flush the tank if it was more than empty. Would have to experiment if the act of flushing fools the level sensor into believing the tank is more than empty...but that is addressable within the automation controller to dampen the leveling input...ie: use the average of the last minute's level data to establish liquid level percent..
For me, I would ensure the valve is electrically closed, and then use a DPDT electric switch to disable the power feed that feeds both the water valve and the waste valves.
I have done similar to automate the winterizing of my S&B irrigation system electrically coordinating an Air solenoid and the irrigation zone water valves to push the water out of the system. Same could be done to winterize an RV coach (at least the air portion). Still recommend antifreeze as a final step towards winterizing.