240 gallon tanks only holding 170 gallons

This would be my guess

 

Ridgeline, you are correct, not to mention that as soon as you park the truck on a slight sideways incline fuel will pour out. But, I don't think the OP understands any of this:

 

Want to add, if you figure your mileage on paper and have a 30/60/90 day average all you have to do is watch your odometer and not your fuel gauge. The last two months my left tank never drops off full and my right tank drops to empty and sets off the low fuel alarm but I haven’t had time to fix it. I know I can run 1200 miles without any issue and that’s how I plan my fuel stops.

 

The other thing is those capacity amounts stamped on the tanks are dimensional capacities.

 

Fuel gauges are not that accurate. Even though it says empty obviously it's not or the engine would stop. You probably have at least 20 gallons left in each tank when the gauge reaches the big E.
Just run it down to wherever you feel comfortable then put more fuel in. It doesn't change your fuel economy.
You'll learn how much it takes to fill at whatever mark it says.
The only time I try to stretch the distance out of mine is if I know a place to buy fuel cheaper. Otherwise I prefer to keep it above the 1/2 mark.