What is the fuel temperature in the tanks during a flight?
Quite cold, in fact.  External air temps at altitude reach -30 to -40 deg C.  The fuel in the tanks wil cool down and reach those temps within a couple of hours exposure.  Jet A has an icing inhibitor added to it to prevent freezing of the fuel in the tanks during extended periods in the stratosphere.

From a heat transfer standpoint, I would probably analyze this as a rectangular bar made from a composite sandwich of aluminum with a Jet-A center subject to convective cooling by air moving at a typical true airspeed, say 450KTAS at STP for FL250.  For simplicity we would ignore compressibility effects.  That may be where you erred.