when I use my sink, or shower the gray water tank starts seems to gets pressurized with air. The tank is not full, but it will start to drain very slowly or even stop draining. I have to open the gray water drain valve to relieve the water/air pressure that gets built up in the tank. I do not think the gray water tank has a vent like the black water tank. when I dry camp this becomes a problem. I have a 2004 Trail-Lite Class C.