Interested to know what the problem is because I might have a similar issue.
Since purchasing the truck, (~2800 miles as of me writing this) I have never been able to put in the 21.1 gallons of fuel that the tank is rated for. The most I have gotten into the tank according to the gas pump is 17.6 gallons of fuel. This is with the orange low fuel light on, and the DTE showing "----".
It is my belief that most car manufacturers put a margin of error in their fuel readout to avoid people running their tanks dry, but I wouldn't think it be be 3.5 gallons.
I was thinking it just might be a bad sensor, or needing a recalibration, but just haven't had a chance to get to the dealer.
Bump for your findings.
J