A while ago, my gauge was sticking at about 11 gallons (as verified by Ibus app) and would not go below that level. I almost got stranded on a long trip by not realizing it in time but as luck would have it I realized that I could not possibly be getting near 500 mile range pulling a nearly 4000# trailer.
It turns out that the driver side level sensor was sticking and if I gave a few dood whacks on the top of the cap housing it would fall and go to the proper level.
I did the o-ring swap to the bigger one when I changed out the level sensor.
Mike