OK, here is what I found out about your DEF and freezing. I found this on Google.com From what I am reading you don't need a heated tank. Sounds like
you are being charged $780 bucks for something you don't need.
Q. When will DEF freeze ? A. This 32.5% solution will begin to freeze at 12 deg F (-11 deg C). At 32.5%, both the urea and water will freeze at the same rate, ensuring that as it thaws, the fluid does not become diluted, or over concentrated. Unthawing will not cause harm to the product.
Q. How much does DEF weigh? A. Approximately 9 pounds per gallon.
Q. How do I keep the DEF from freezing in the tank of a Truck? A. SCR systems are designed to provide heating for the DEF tank and supply lines. If DEF freezes when the vehicle is shut down, start up and normal operation of the vehicle will not be inhibited. The SCR heating system is designed to quickly return the DEF to liquid form.