I have just had this extended stay trailer delivered to my seasonal site and would like to understand how the 2 grey water tanks work. I have a black water valve and a grey water valve. I understand one tank is for the kitchen and one for the bathroom which includes a hook up for washer/dryer. I have the grey water tank open and the black closed. How does one make sure both tanks empty. I have seen ref. to 3 valves but this one only has 2 at the sewer junction. Thanks.