Def reset

[Toops]

I put a 2-1/2 gal jug of DEF in my 2020 GMC 6.6L diesel the other day and drove it 1,000 miles and had to put in another 2-1/2 gal jug in to get back home. got home and its reading a little over a quarter and says def is good for another 114 miles. doesn't seem right. A full def tank ran me over 5,000 limes when I first got it. Why is it using so much now? Will elevation change make that much difference? I'm going to fill the DEF tank up today. When I do will it read good till 5,000 miles or will it just say nothing and read full on the gauge? Thanks