1/4 cup of bleach for every 15 gallons of water that your fresh water tank will hold.
Let the solution sit over night in the tank.
Using the water pump run the mix thru every faucet the unit has for a few gallons each, outside shower?
Be sure to run some out the low point drains.
After the tank is empty refill and drain till you can no longer smell bleach.
Check the water pump bowl screen and faucet aerators for debris.
Remove the water heater anode rod, flush the tank out.