This question comes up quite often, so let’s start:
Assuming it was winterized by putting the “pink” stuff, aka RV Anti-freeze through the pump into all of your water lines.
First, add water to the freshwater tank, don’t need alot, just enough to run all of the faucets, I do about half tank(you will see why later). Notice we’re doing NOTHING with the water heater, assuming winterization was done correctly, it is on bypass and no pink stuff is in there anyways.
Second, turn on the water pump, go through each faucet both hot and cold and run it until pink stuff doesn’t show anymore and it runs clear. Don’t forget outside kitchens, showers, etc. If you have ice maker, you will need to look in your owner manual how to deal with that, as well as washing machines.
Third, once you’ve done all of that and everything is running with plain water, it is time to sanitize the system.
Here is blog post on sanitation of freshwater system, scroll down to Sanitize section and continue from there(notice we’re still not doing ANYTHING with the water heater).