OK, the most common cause of this is the nylon seal has a small crack which will require a NEW valve. ALL LP tanks you always open all the way to snug for use and obviously TIGHT when you shut it off. I am not condoning what I state now. Can you open the valve all the way and get it snug and the leak stops? Sometimes they will. If NOT, you need to close the valve and take to a LP dealer and have them drain the tank and replace the valve. You need to have this done regardless if the first suggestion works for this trip. Doug