They're supposed to be good for 5 years, and I have the same coach (2015 age) and replaced mine around 5 years as it was blinking the ageout code. I would not think it is necessarily defective.
They do alert on propane as well, so I would do a little troubleshooting first. Turn the propane off while in storage, see if that makes a difference (I also had a propane host begin to leak at 3 years, so this is a real possibility. You can inspect with a soapy solution brushed on to the hose connections, hose and fittings). I would also check you are not overfilling your batteries to avoid boiling off. They should only filled with distilled water, just to the point that they are just touching the very bottom of that plastic piece inserted.
The sensor is 12v, so verify your house battery voltage is at least 12.0 volts or greater.
Also, if/when you go the replacement route, the new sensors are a different size than the original. I cut and stained a thin wood template that covers the whole, and the new one fits into. Got the wood home depot, think it was 1/4" thick.
DaveB, Raleigh, NC
2015 Tiffin RED 33AA, w/Honda CRV
VMSpc, Magnum BMK/ARC50