As Pete has suggested you need to determine which wires run the elevation motor and which wires run the recoil motor. The recoil circuit consist of three wires. A negative and positive feed to the motor with the negative side routed through a micro switch . This is connected to a constant power source. Directly to the battery would be fine.

The third wire is a trigger wire, and is a user activated negative wire. On an RX 18 this would be the airsoft wire. You will have to find this source on the SLU. I have no experience with the SLU so I can't help you there, Because it is running a motor you need to be careful the circuit can handle the load of an electrical motor IF the SLU can trigger an airsoft gun, that that should become your trigger wire for the Recoil unit. I hope this helps. Sorry I couldn't give you more specific instructions.