I have thought about using gray water to flush the toilet, but after studying my setup, concluded that the effort did work out.
In my case, I only have 90 gallons of fresh water. A 55 gallon gray water tank and a 50 gallon black. In use, this would allow for me to use about 15 more gallons for fresh water before filling both the holding tanks. Not enough in my opinion to install a new pump and other hardware.
When boon-docking or dry camping I install a twist on valve like this.
Keeping both the gray and black tanks closed. When the gray tank is about half full, I will close the valve on the twist on and open both the gray and black. This will equalized both the tanks. Then repeat latter on as needed.
This may not work of all RV's but works great for me.