Since I don't know what you've tried and haven't tried, I don't know if this will be helpful, but I copied the Carrier unit and changed some stuff:

Basics tab

   - Unit Classification: Sea  (this is needed so that the planes which are "fueling" can load and unload as they please, even when the air carrier is "flying" - if you classify it as an air unit then you won't be able to load/unload properly because air units only load/unload while "grounded" - i.e, when they are hosted by something themselves. The lack of distinction between being "on the ground" and being hosted by something doesn't matter in the vanilla game because there are no situations where an air unit would be able to load something while flying, but it becomes important here because hosting a unit is the only way to replenish "fuel")

   - Moves Like: Air (you'll want to make sure it can move over all tiles)

   - Unless you want the air tanker to have infinite fuel itself, you should specify some movement range, and you'll probably want it to move at a more plane-like speed than two tiles per turn

Transport tab

   - Transporter: Yes

   - Capacity: Some number above 0 (however many planes you want to be able to "land" for fueling at once)

   - Transportable units: Whatever units you want to be able to be refueled at the air tanker

Other tabs aren't strictly needed but you'll probably want to mess with the production cost and combat statistics to make the unit better fit whatever game balance you've established with the other units you have.

Then you have a unit that works the same as a carrier, but flying instead of sailing, and presumably with a reduced capacity and finite range. I'm not sure anything better can be done unless the game is changed to look at fuel as a substance that is quantifiable, consumable, and transferable instead of as a number of turns a unit can survive unhosted.