Ok, then I think, I know, what happens.
LT and RT are both analogue axis, so you can assign them to the tiller. BUT: LT only steers to the left, while RT only steers to the right. Since you can assign only one axis to the tiller, you will only steer in one direction and have no chance to put the tiller in neutral position again.
You need a control with a regular axis, preferable one, that returns automatically to neutral position for controlling the tiller.