I am not a big fan of Xinex leak detectors. The sensor element has a finite life of about one year and when it fails it gives false positives. I believe that a propane solenoid, used rigorously plus periodic pressure leak down checks, makes a propane stove safe.

But if you have to have one, connect it to the solenoid breaker. It will give you an extra indication of leaks when the solenoid is supplying gas.

And think about it, if you were to wire it independently, when the solenoid is off any leak is going to quickly drop the pressure in the lines downstream of the solenoid making the leak go away and making the sensor worthless until you turn the solenoid on and pressure up the line. In other words, the leak detector will only find a leak when the solenoid is on, so why not wire it to the solenoid power supply.

David