How to Save Steering Trim for a Skid Steering boat

Hi, for a Skid Steering boat ,maybe here is a mistake in this boat , in mannul mode it can’t go stright , i set servo2 to 73 servo3 to 74 , I have read the DOC in Tuning Pivot Turns — Rover documentation (ardupilot.org) but i don’t know , where the turning value stored ?