Just call them freighters.
And I would say probably not. The more fuel you carry, the heavier you are, the slower your are, and the more fuel you consume. That is why racing cars do not carry fuel for the whole race, but carry as little as possible, and just refuel in pit stops. They are faster that way over the whole race.
In the same vein, I would think that freighters only carry enough fuel for the planned distance.