Please help with channel mapping Spektrum to ArduRover

The Spektrum transmitter outputs the channels as:

Channel 1 = throttle
Channel 2 = Aileron
Channel 3 = Elevator
Channel 4 = Rudder

Since this is different than the default ArduRover convention I thought to change the values in the RCMAP configuration:

RCMAP_PITCH = 3
RCMAP_ROLL = 2
RCMAP_THROTTLE = 1
RCMAP_YAW = 4

To which channel do I assign the “Ground Steering” value? Does it have to be the ROLL channel?

What I really want is to have throttle and steering on the left stick, and two servos for a camera pan and tilt device controlled by the right stick.

Thank you,

Paul