Dear Nabil,
I assume that by auxiliary heater, you mean "fossil backup," or supplementary heat from an auxiliary gas system. As long as the "fossil fill fraction" under "Dispatch Control" on the Thermal Storage page is zero for all periods, the model will not fossil backup.
The tank heater inputs are for freeze protection to prevent the HTF in the storage tanks from solidifying at night. SAM assumes that the tank heaters are electric. That is intended for molten salt as the storage medium. If you use an oil, the fluid temperature is not likely to reach the oil's freeze point. You can check the results to see if any power is consumed for TES freeze protection: The HTF temperatures are listed as "TES HTF temperature..." and the freeze protection power is listed as "Parasitic thermal field freeze protection."
Best regards,
Paul.