Am I correct to understand that both the turret rotation and the gun elvation run through the same ServoMorph? if so, that is your issue. When your turret rotates in one direction, and the guns elevate, the servos are sharing the signal in one dection from neutral. When they return to center line the servos return to neutral. when you then command rotation in the opposite direction, you get the correct rotation because the train svervo can do so, BUT the elevation servo, having been at neutral is now attempting to move your gun elevation in the opposite(down) direction. What needs to happen is that the elevation servo needs to move from neutral to up, regardless of the turret's rotation direction. Unless you have a digital radio that will alow you to write script, I do not think you can do it, at least not through a shared ServoMorph.
If I am not mistaken, the ServoMorph will allow modification of the servo speed and the limit of travel. But unless it will reverse an output, selectively, you are stuck.If you have a spare channel available, you could out elevation on it separately, or go with a digital radio, like a FrSky that allows scripting. Or, as Umi suggested, use a cam.