Freezing Temps And Tanks Not Draining

What I think happened was that a small amount of water went into the grey tank, froze and the anti-freeze and additional water was sitting above it. No, we don't leave the tanks open. The discharge pipe, attached to the trailer, not the sewer hose, was the issue.

Thanks everyone. Will test out the Reflectix and drop light Tuesday night when we have another hard freeze.