[neon_rt]

Have you done any mods to your engine?
It is possible that the computer made an adjustment to the fuel map while you were towing, in that case, after a while the mileage would return to where is was. Data is sampled and stored and the data is used cumulatively to determine fuel mixture when you are in open loop mode. Any adjustments that the computer made will eventually be replaced by newer ones as you drive in closed loop mode.