The function of your waste holding tanks should have been explained to you during your walk-through at the dealer. In the absence of that info - you can go to Keystone's website and look up the specs on your trailer to determine the number and size of your waste tanks. There are many different configurations. For example: Our Cougar has one 38 gallon black water tank that holds waste from the toilet. We have a 38 gallon grey water tank that holds waste water from the shower and bathroom sink and there is a 38 gallon grey water tank that is plumbed into the galley sink. There is a dump valve for each of these tanks but they discharge through a common waste pipe/fitting. The sensors on these tanks are known to be very temperamental and inaccurate. Therefore, the tank you think is full may not actually be full. Have you successfully dumped these tanks? You might try reaching out to your dealer for assistance - good luck
