The DEF tank has a heater probe in it. If you are running, the DEF being used should not be frozen. If it does you have other problems. I think what I remember from the Camp Frieghtliner class is, when the block heater is on prior to engine start you are also turning on the DEF heater.
Dan Sees, Full Timing in the MH & Part timing in a S&B In Hampstead, NC
2013 Winnebago Journey 42e, 2014 Featherlite Car Hauler 3110 17.5', 2008 Mazda MX5,
2008 Toyota FJ Cruiser