Use what is in the tank. Or, if you question how clean that water is, drain it and put fresh water in it. The general recommendation is to use 1/4 cup of bleach per 15 gallons of water. Be sure the water heater is turned off. Add the bleach to a gallon of water and pour into the fresh water tank. Run water through all faucets until you can smell the bleach. If possible, drive the RV around for a mile or 2 so that it sloshes around in the tank. Let the bleach / water sit over night.
Drain the water heater and fresh water tank. Add fresh water and flush all the lines. You may have to drain the water heater and fresh water tanks more than once to get rid of the bleach smell.
2015 Solaire 201SS
2015 GMC 2500HD Duramax