Your mileage is about right. Next time check the “avg. speed” during that same tank of gas. If it is low (25 mph), then you are likely doing a lot of stop and go driving which explains the 14/15 mpg. If it is higher, say 30-35, you are having more continuous driving and are around 18/19 mpg.
These are big, heavy vehicles. Still not sure how people are getting over 20 mpg in “city” driving.