We have a 2006 Horizon, and sometimes one slide lock or the other will stop working. Here is what I have found. if you remove the black plastic cover in the overhead bin you will find the lock motor, with a screw shaft, and 2 limit switches. the motor is fastened with a pin and lock pin. remove the lock pin and the anchor pin, and let the motor drop a little. then have someone just bump the control switch. bump lock if it's in the unlocked state or unlock if it's in the locked state. if it activates, reinstall the motor pin and lock pin.
Here is how to keep it from happening again. Each time you either lock or unlock, at the end of the lock travel bump the switch the other way. This keeps the screw shaft from stopping hard against the limit switches.
Works for me.