I just did a round trip from Salt Lake City to San Diego, towing a 7x16 cargo trailer. My MPG ranged from 9-12 when towing at 75MPH and I got around 15 when towing at 55-60MPH. I did not calculate DEF usage, but it was horrible. I assumed that if I topped off before the trip that I would make it back, but not even close. When I got to SD the warnings started to appear, so I added 2.5 gallons of DEF, the gauge then read full. On the way back, more steep uphill climbs, I was not taking it easy, I wanted to really test the LM2, by Mesquite NV more warnings and the gauge showed one red bar. Assuming that the tank was empty I purchased two 2.5 gal. jugs at Walmart and after about 3 gallons in the tank overflowed. Arriving back home the gauge was down a only a few bars (1/2 to 3/4 tank), but after taking it out of tow mode and putting a few miles on it the warnings came back. I added my remaining DEF from the overflow situation and still had to run errands all day before the thing realized there was enough DEF or the estimated miles to empty DEF was back to normal.
My take-away, the "calculated" gauge and warnings based on estimated miles of DEF are a joke. The consumption seems reasonable but whats the point of a 5+ gallon DEF tank that you only get to put 3 gallons into before the truck turns itself into a paperweight... Seems like a flaw or bad programming or bad design from GM, hopefully they realize this and a recall on the tank/sensor or a software update is coming. Next long towing trip, I'm packing a couple jugs of BlueDEF. and I'll keep my speed closer to 70MPH and keep my foot out of it on the hills.
Overall, wow, the power of this 3L DMAX (backed by the 10L80) is impressive, no drama towing, just set the cruise and watch for the other dummies on the road.