Part of the problem with DEF gauges is that you can't use a float to meter the level like you can fuel, because of the freezing temperature of DEF. They instead use sensors placed at various levels with computer interpolation of the levels between sensors. In the RAM, the gauge isn't really accurate until it drops to about 50%.
They also go through regens. One of the telltales is a strong diesel smell coming through the vents when you are stopped at an intersection. If you install something like an Insight CTS monitor, you will see a small red "R" in the top of the display when in a regen. Empty with a lot of city driving they seem to happen about every 800 to 1000 miles.
Empty it seems to be about 1000 or more miles per gallon of DEF. I'd agree that you tank wasn't really full.