On our Four Winds, there is a fresh water tank drain spigot on the outside of the coach. A cheap looking thing of the type that you might find on a ice cooler. You can drain the antifreeze from the fresh water tank from that spigot.
Then we just return the bypass valves on the water heater to normal, put the heater's drain plug back in, fill the fresh water tank and open all of the faucets and run water until the water is clear - and then double the run time.
Of course, depending on where you live and if you can dump clean water on the ground from the grey tank will determine how much water you run through it. No need to run the toilet, etc. or dump the black tank until you start using it.
May or may not be the best way, but it is how we used to do it with our boat and now with our RV.
There are sanitizing products on the market as well as some people use bleach or chlorine. But we have found (from our boating experience) that if you use the fresh water tank often enough, you won't usually have much problem with stale water.
Even then, we typically use fresh water for washing dishes, bathing, etc. but not drinking. We always take bottled gallon jugs of water for coffee, making ice cubes, and so on.