The trouble with continous servos i have found is the neutral spot where they don't move is very small.

Just tie this into you receiver independent of the RX18 You can use a servo Y cable and hook it up in parallel to the BARC. This will allow up and down movment of the bbl as the reciever controls the servo not the RX18 or the BARC..

This is true for any system that uses standard RC. Tank people seem to forget that receivers are designed to move servos back and forth and if this is what you are looking to do you don't need to run it through a Tank control box. I think this comes about because of the proprietory radios on factory tanks. They don't use servos so people who are introduced to RC this way haven't been exposed to servos in use like they were desgned for.

So a regular reciever can easily control turret rotation and gun elevation. using continous servos