Cedar Creek gray water tanks

Do you know where (height) the various tanks are located? Most dropped frame units have a black and a grey in the dropped area and the third (or more) grey is located in the upper frame area. If you use a Valtera gate valve to equalize the upper and lower grey you might be asking for trouble. Simple physics say liquid will flow to the lowest point (the grey tank in the dropped frame area) until it is completely filled, then it will back up into the upper grey tank. Which is fine in concept. But in reality the pressure of a full tank plus the pressure from the upper tank is likely to pop the top on the low bid tank used by many manufacturers.