You didn't mention if you were having to use the furnace because of cool temps, but............we experienced a strange alarm sound recently when we were dry camping in a Walmart parking lot and had to use the furnace overnight. Turns out our house batteries were very low, as far as voltage goes, and I found there was a weak alarm sound coming from the base of the furnace unit. After confirming the status panel, I found the battery voltage very low and once I started the rig to charge the house batteries, the alarm stopped.
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)