Unreliable DEF tank monitor in our GMC Sierra 3500 HD?

[AbHDToyHauler]

My 2018 came up pretty quick too when i first purchased it. I am pretty sure the dealership only put in enough to get the warning off. Once I topped it up it has been "normal" and I guess accurate. Obviously it calculates the def useage based on fuel used as when towing it uses it much faster . Even when its low if you read the message carefully it tells you that if you don't put fluid in you will have like a 600 miles before it goes into limp mode. (Don't quote me on the mileage to limp mode as i'm pulling it from memory and my memory is awful).