I know all trucks and brands are not the same. But when I got my Chevy Diesel 3500 Duramax (New), it initially used (what I though) was an excessive amount of DEF. The first fill up took me completely off guard. We were heading for a State Park (Northern Indiana), when the light came on. I was lucky to find a GM dealership still open and they had one jug of DEF left. They even put it in for me, as I'd never done it before. The guy at the dealership told me, when it came from the factory, it wasn't full. The dealer I purchased it from probably missed to fill it too. So, the little bit the factory put in, expired rather quickly. Although my dealer did give me a full tank of diesel.
But the second fill up came much quicker than expected also, and by now, I was beginning to wonder what kind of strange animal did I just purchase. So, I started tracking the actual miles.
Little by little, the distance between fill ups got longer and longer. I don't know why? Maybe there is a break-in period for DEF also? Who knows?
But I'm glad to report, I use 5 gallons every 8000 miles now.