....the semi I used to drive averaged about 6 mpg but had a range exceeding 1100 miles at mostly highway speeds. I never ran it dry and usually filled it about once per day at somewhere around 500 miles. Its secret to long range was simple - two saddle tanks holding over 100 gallons each.
Ah...so you carried an extra one hundred gallons(700 pounds of diesel) that you never used. If you would have carried 100 fewer gallons, your mpg would have been higher.
One of the things I missed about owning a diesel was the ability to vent and top off to the tippy top with no worries allowing for a 14 gallon tank in my VW TDI to hold almost 15 gallons. This also allowed for the most precise measurements of mpg since one can see the fuel right there at the cap and one sees it each time at the same spot, knowing the possible filling error has been eliminated.
However, diesel fuel, like most liquids can and will expand when heated up, i.e like parking out in the sun as the earth warms up during the day, so the only worry regarding a diesel and topping up, as long as one tops up carefully while watching the fuel level at the pump so as not to inadvertently spill it on the pavement, would be if you vented and topped up and then didn't drive far enough on a cool morning before parking to burn off a little bit of fuel. In that scenario, some fuel could spill out around the cap or overflow system in the vehicle, but that would be a rare scenario for most drivers.
I always brim the tank as you explained, best way to get accurate results, although we are relying on the pump calibration for that, even though we know we get the same amount in every time as you can see the fuel in the filler, the reading on the pump might not be that accurate.
In my xB if I stop at the first click I still have a gallon of space in the 11.9 gallon tank and since I don't have a very big tank and don't want to test the 1.9 gallons reserve when the low gas light comes on I try to fill it pretty full. That last gallon can take about 5 more minutes of pump a little and wait for it to flow into the tank but it's like 10% more fuel in the tank since I have to start thinking about getting more gas when the light comes on at pretty much exactly 10 gallons used since full full. I do notice quite a variation on how accurate the pump gallons match the scanguage gallons used but once in a while I find a gas station pump that is within a ten of a gallon fillup after fillup. One thing that bugs me is how the scanguage continues to "use" gas after the ignition is turned off especially if I rev the engine just before I turn off the ignition. Why? Because flushing the engine with fresh air actually makes it start a lot easier next time ... Like on the second compression stroke.
It's also dangerous for planes to land with more weight than necessary, in an emergency situation when the plane has not long been in flight, they have to dump the fuel before landing or the plane will break up if it's fully laden with fuel.