In regards to the dash computer - There are a couple V8's in my family and I think the dash just lies. Always has, always will. My theory is that it's related to the way they handle averaging during coast. The computer is sampling a number of readings and doing some math for distance traveled divided by fuel used per unit of time. When coasting, your traveled distance per time unit is rather high, and your fuel consumption per time unit is very low. However, I'll bet darn near anything that during the coasting, the yoda is assuming way too close to zero for fuel consumption. Like, near divide by zero (which equals infinity!), and this is used for the average just like any other driving condition. This artificially inflates the measurement.
I have noticed that some other cars (audi for example) do not do this for a true coast condition, nor for a true idle condition. Instead, they seem to go into some sort of "low fuel consumption" mode where the math is different and doesn't have a big effect on the average (like the toyota). I have an audi A6 that always calculates within 5% of my actual fuel usage.
In regards to economy in general -
Different types of fuel will have some effect, I would assume that a higher octane fuel will provide slightly better fuel economy, especially in hot climates.
If in doubt, check all the critical parameters - look at your spark timing, fuel pressure, critical engine sensors under load. If you have a friend with an identical T4R with same emissions standard, do a comparison.
I have heard rumor of the additives in some fuel causing restriction in the little thimble filters that are in the input of every fuel injector. This could result in poor fuel atomization, which would cause a number of problems including reduced fuel economy. There are lots of guys who do fuel injector cleaning, so if you can find one nearby, it's always possible to pull your injectors and have them flow and spray pattern tested.
If everything checks out - I think it might be possible that our trucks might just be really thirsty, all the time. Keep in mind that our fuel tanks are small and even a slight difference in level might result in a different top-off volume. For best results during fuel economy trials, always refuel at the same fuel pump, with the tires in the same spot, at the same ambient temperature, with the same pretty attendant at the cashier counter.