I am very familiar with driving this truck. I've driven it 100,000 miles in the last year. It has never acted like this until now.
When I said it filled 25 gallons, I mean that there was still 6 gallons left in the tank. And ALL the miles I put on it at that time were highway miles. I never idled it. That's why I'm worried something is very wrong. A 31 gallon tank, even with the computing, should NEVER read that it has only 1/4 left when only 80 miles of highway driving has passed. Again, when that happened, it ever so slowly again inched its way back up.
Notice how I never say "range", which I totally understand, as that can be calculated based on my driving habits. But what should NOT be calculated is actual fuel. If I put 25 gallons in a tank that is almost empty, it should show that it is full. Range, sure, calculate it. But not the actual gauge.
In my truck, I have found that the gauge is directly tied to the range. When the computer calculates that there is a lower range, it will decrease the fuel level. To me, that is strange behavior, but it's exactly what's happening.
Final note, this just started happening. Hasn't happened since I bought the truck.
