Here's an example of how DEF useage is tied to fuel useage.
Pulling my boat 460 miles round trip, 10.9mpg.
Just under half a tank of DEF left and only 498 miles estimated DEF range.
By the end of the week it will be back to normal but once its under 500 miles it won't reset until you add DEF. After adding 2.5 gallons it still only estimated 1100 mile range but should show around 4500 after a few days of non-towing.