Did some research and spoke with Freightliner. The rig pulls from the drivers side tank which is where the sender is. The tanks gravity feed together. This is what Freightliner told me. To make the gauge function properly was told to always fill the passenger tank first. Seemed doubtful to me.
So I tried it with just a short trip and the gauge seemed accurate.
We just took our first trip of the year. Filled up, passenger side first and off we went. Stopped after 420 miles and was reading barely under 1/2 tank. Filled up with the passenger tank taking 20 gallons, drivers tank exactly 15 gallon. Both fill-ups I let the foam settle and filled exactly to the bottom of the neck to be accurate. So right at 12mpg with the cruise set at 64mph, 95psi in the tires.
Not bucking any wind or hills.
Filled again the same way and went another 70 miles and the gauge just came off full. Looks like our capacity is right at 70 gallons? So try filling the passenger side first and see what happens. So far so good with us.