X2 on weak battery. Also, it will alarm sometimes when first turning on the furnace . Alarm will reset after battery charged, but the alarm also has a lifetime (it will keep alarming when it reaches it, it can be reset, but will only silence for 3 days). Check the date on the alarm. I'm not home right now, so I can't check the lifetime of it (I think it's 5 or 7 years). If it is outdated, you can order fron Amazon, or pick up one in Camping World.
Got this from their site:
How often should safety alarms be replaced on a RV?
Replace your Propane and CO Alarms that are over 5 years old.*Only install Propane and CO alarms that are UL listed for Recreational Vehicle Use. Some CO alarms or their replaceable CO sensors may have a shorter service life. Check the manufacturer's owner's manual for replacement information. Replace your Smoke & Fire Alarms that are over 10 years old.*