In the middle of the night, we were awakened by an intermittent "beep" coming from our living room. It's the RV Propane Alarm. Apparently it needs to be replaced every 60 months, & since we bought this used, we had no idea what the expiration date was. I pulled the unit & it's dated Oct. 1 2016. The model # is 30-4420-P. I cannot find one locally (Tampa), even at an RV joint. There ARE a couple of local places that carry the 35 Series units. They are a little more costly, but are they plug-and-play interchangeable? Thanks.