I think you're right. Saw this...each US gallon (3.785 litres) of the fluid will weigh 9.2 lbs (4.17 kilograms) or 4170 grams. Then found this:
Specific Gravity: 9.1 (32.5%) or 9.4 (40%) lb/gal @ 60°F (1.09 – 1.13 g/cc)
This seems more accurate so lets use this:
5.3 Gallons is 21876.76 grams
Divide by 1.8 (your current useage)= 12,153.76 km
12153.76/1.609=rounds to 7554 miles
But that's draining to empty.
Based on your consumption rate of 1.8g/km that works out to .7 gallons remaining at 1000 miles and .21 gallons remaining at 300 miles.
Also that consumption rate looks like it's based off that ignition/drive cycle only and is not indicative of your cumulative consumption.
https://www.trucknews.com/features/diesel-exhaust-fluid-def-facts-unveiled-at-tmc/