Even when I am not using it!
I was picking up the garbage tonight when I heard a "beeping" coming from the RV. Going in, I noticed the light on the propane detector was flashing on and off telling me it was time to replace it. I checked back in the Blog and saw I had last replaced it in April 2016, so it lasted exactly five years.
They are $100 US (plus $16 shipping) on Ebay or $147 Canadian. I will check around locally before I order one. In the meantime I took it off the kick panel below the sink and cut a wire. No more beeping until it is replaced.