Thank you for taking the time to post your experience. As much as I don't own a Duramax I am a new owner of a 2013 Ram 3500. This is my first truck with DEF so I am also very inexperienced with the ins and outs of the DEF issue.
I only have 2500 kms (approximately 1500 miles) on my truck so far and I topped off the DEF tank recently from a bulk tank at a truck stop. It took 13 litres, or roughly 3 gallons of DEF. I was quite surprised at how much it took, but I think that it was not full when I bought it new from the dealer. The DEF gauge was not on full when I picked up the truck and I have read that the gauge reading is inaccurate at best on these trucks. I will see how it does now that I know the DEF tank is full.
I must say though that the lack of diesel exhaust smell and soot all over my garage is very nice!
2013 Ram 3500 Megacab DRW Laramie 4x4, 6.7L Cummins, G56, 3.73, Maximum Steel, black lthr, B&W RVK3670 hitch, Retrax, Linex, and a bunch of options incl. cargo camera
2008 Corsair Excella Platinum 34.5 CKTS fifth wheel with winter package & disc brakes