Help with a CAT 3116 Computer

I have a 1998 GMC Cabover with a CAT 3116 with refrig box on frame. I have been driving with a fuel pre-heater catalyst and an HHO system on the engine. My fuel mileage has improved tremendously. However, the fuel mileage gains return to the baseline after a couple of days of driving. When I reset the computer the gains return. Then again after a couple of days of driving the fuel mileage gains go away. I need to know why the computer is relearning? There are no check engine lights. I need to know how can I keep the fuel mileage gains without having to reset the computer every couple of days. Any answers would be helpful!