GM specks that ride height measurements be taken with a full tank of gas, so that's not the problem.
My truck (2016 LT) was an inch lower on the driver's side than the passenger side. After driving it about 5000 miles the difference was down to about 5/8 inch. Seems it was adjusting itself. Before I installed the rear anti-sway bar two weeks ago I loosened all six of the rear spring mount bolts. I then measured ride height and found that left to right was equal to as close as I could measure (less than 1/8 inch difference). I bounced the suspension around a bit to settle everything, remeasured to make sure it was still level and retorqued the six spring mount bolts. The truck sets level now. It appears the problem was nothing more than binding spring bushings. I suspect that is the cause of most of these problems. It's an easy fix that can be done in about half an hour, even if you're an old codger like myself.