I'm going to guess that you have an ice clog in the line either between the tank and the valve, or from the valve to the cap. You're going to have to find a way to get some heat into that area of the underbelly to thaw it out.
Can you pull down part of the underbelly and stick a 60 w light bulb up there to add some heat? Another idea if you have one of those oil filled space heaters you can place it under the trailer near the valve and let it heat it through the underbelly from the bottom. You might need to find a way to block the space off though so your heat doesn't blow away.