On my 2015 RAM once I flip the valve on the bed tank the fuel gauge starts going up and so does the "miles to empty". I'm guessing the RAM boys figured out a way for the computer to re figure the "milage to empty" including the rate at which I'm burning fuel at the time (pulling a load vs NOT pulling a load).
Although I don't have accurate knowledge of how much fuel is in my bed tank (once I tap into it) I DO have an EXACT reading as to what I have in my truck tank and can make the proper estimate for when I to fuel.
I would have to drive way longer than I'd like to in a day to burn through both tanks and have to stop mid-trip. That's what makes having the bed tank so appealing to me, we can get situated in a campground, drop the trailer and I can head to any nearby station for fuel without having to deal with navigating with the trailer (unless I choose to do so).
Dan