Hookway,
Port side behind the wheel you will find a the locker for the in use tank. You will notice a solenoid and propane hose. Port side in front of the in use locker is a second large locker used to store a second not in use tank. The not in use tank is stored in the aft of the locker. All lockers should be insulated where the covers close. You should check the vent is not blocked, it is in the bottom of the lockers. Enjoy your new 400. I have had mine since 2000.
Nick