I have a 2020 Sierra 1500 AT4 with the 3.0 Duramax. I just got back from a 2500 mile trip pulling my 27" RV about 6500 lbs. I used 15 gallons of DEF. This seems a lot! the pictue below with miles at 4116 the DEF was Full. At 4475 miles I was almost out. I took my truck to my local GMC Dealership when I got back. They can't seem to find anything wrong with the engine. Does anybody have similar issues? Is this normal? Thank You..