[RMcNeal]

Exactly what Dick said. I would add, the thing that seems to freeze up first is the short section of sewer pipe below the rig outside the coroplast. If you can somehow enclose that, even with cardboard and run a small electric heater to help it thaw, it may speed up how quickly you can get the tanks drained. Even wrapping an old blanket or sleeping bag around it will help some.