Mine has done this since day one. It sounds like I have the same valve that the OP has. We live in our trailer full time so I didn't take it back to get it fixed. My (temporary) solution has been to use the water pump when taking showers to pump the tank down a bit so it doesn't overflow out of the vent line.
Permanent solution will require taking the chloroplast off to attempt a permanent repair...I'm inherently lazy...temporary solution has been working just fine!
