1975 hoss

I now have a 20 Denali and came from an 18 High country. Same engine technically. Just went on a 2700 mile trip towing my camper and I went through 3 1/2 tanks of def. My 18 never went through it like this. I could get about 4-7k miles out of a tank before. I took it to the dealer and they said it will use more because of regulations. I said not 5-6 times more. I used roughly 270 gal of fuel and 24ish gal of def. this is ridiculous..... dealer won’t do anything for me.