This is actually the answer I was hoping to hear. Your vehicle is set up incorrectly for skid steering.
First, set your Sabretooth driver to independent mode via the DIP switches.
Next, set ArduPilot to use ThrottleLeft and ThrottleRight outputs. Ensure you have motor direction set correctly by using Mission Planner’s motor test page. See here.
Lastly, see this link for the steering mix. PILOT_STEER_TYPE=0 should result in the same RC steering behavior as you have now.