I said I 'considered' the long bed. I have the short, 25 gallon tank. I always get a receipt, jot my trip mileage on it and reset. The light came on at about the 360 mile mark, I filled up at 378.9, I squeezed in 21.164 gallons.
That's 17.9 MPG, had one snow day on this tank other than that pretty warm for February.
There is usually a little less than 4 gallons in it. I guess that is enough to protect the fuel pump.
As your tank is longer, you might need a little more in there to protect the pump than a 25 gal tank. Maybe this is why your light is so soon?
At any rate I would trust the light and assume no harm no foul unless you ignore it consistently.