DTC = Diagnostic trouble code - a malfunction stored in the ECM. It wouldn't necessarily trigger the MIL, but it might hurt your gas mileage.
Reset the trip odometer when you fill-up. The gas gauges are NOT accurate in these trucks at all, so you cannot possibly know how many gallons have been used to accurately calculate MPG to 2 decimal places.
Here's what I do. I reset my trip odometer at every fill-up. I always fill the tank completely until the automatic shutoff. I get a reciept and record the total vehicle mileage on the reciept. I use the trip odometer and average MPG I've seen over the past few fill-ups to estimate approximately at what mileage I'll need to fill up.
I then record the fuel purchase into an Excel spreadsheet including, date, cost, gallons purchase, total vehicle mileage, etc....
I then calculate the MPG by dividing the miles travelled (trip odometer), by the gallons purchased.
Never top-off your tank, as that will effect your calculations. It will also cause problems with the pressurized fuel system, and create problems with the EVAP system.