Century Series (any unit with fresh tank above floor): Turn off fresh water pump. Open drain valve between the fresh tank and the water pump. Open fresh tank cap to vent. Water will drain under the unit.
Denali/Rainier Series (any unit with in-floor fresh tank): Open drain valve between fresh water pump and the rest of the unit (it is inside to the left of the mechanical room door and is marked “drain”). Open fresh tank cap to vent. Turn on the fresh water pump and it will pump the fresh tank out and drain it under the unit.