Safe-t-Alert LP/CO detector issues

I have a 2010 TrailMini 18L. We've been having issues with our 70-742 detector and false alarms/warnings. We bought our trailer 1 year used and the alarm went off while camping during the night. We woke up, but did not notice any gas smell. Our propane heater was running and it is right next to the detector. We shut off the heater just in case, aired out the trailer, went back to sleep, but the alarm went off again. Didn't get much sleep that night and called the dealer the next morning. They recommended we just cut the wires.

We later had a tune-up/servicing done at our dealer and had them replace the detector with a new one. Now, when in storage in our garage, we can hear an intermittent beep about every 25-30 seconds or sometimes a constant, fast beeping. This seems to happen both on battery power AND when plugged in. We had the batteries replaced at the servicing too. Recently, we pulled it out just to air it out: plugged it in, opened the windows, turned on vent fan, etc. The propane bottles were closed and we did not turn on/use any of the appliances/toilet. The battery power read Good/Full. It was fine at first, but then the intermittent beep started the next day. Reset it, it was fine for awhile, then started up again. Called the dealer who said to bring it in as soon as we could. They're an hour drive from us so it's not an easy thing for us to do, so we closed it up, put it back in the garage, left the vent cover open, left it plugged in, now we hear the fast beeping in the trailer again. We don't park a car in the garage nor store anything in there that would cause the detector to go off.

Anyone else having issues with theirs or have any ideas for me? Safe-t-Alert said to have it tested, but this is already the 2nd unit we've had problems with.

Thanks!