I do things a little different than you.
1. Go inside and open all faucets.
2. Go outside and open low point drains to get as much of the pink stuff as possible to drain out. Close low point drains.
3. As others have said, bleach. Hook up every hose you intend to use this year to your tank fill port (could be gravity or a hose connection) and pour bleach into the hose furthest from the camper. Now hook the hose up and fill the tank. This way everything gets some bleach to kill the bad stuff.
4. Pump bleach water through system to get the pink stuff cleaned out.
5. Turn off the pump. Close/open (depends how you look at it) the bypass for the water heater. Turn on pump to give a short flush of the water heater. Turn off pump and put plug in water heater. Turn on pump and open a hot water valve/faucet to fill the water heater with bleach water.
6. Let sit overnight. Next day. Repeat numbers 1 & 2 plus drain your holding tank and water heater.
7. When ready, fill with fresh water and enjoy.