That is a very optimistic sales associate. Upper trims are usually considered the TRD Sport, Off-Road and the Limited.
Having said that, I'll share a bit about how allocations work. TMNA (Toyota Motors North America) has a build schedule and they tell the regions what models they have earned...
Tank size has no part of the calculation for fuel economy. Fill it up, drive and fill up again. Divide the number of miles between fill-ups by the number of gallons it took.