Sounds about right. I know the "clocks" in those things are far from accurate, but for their purposes, +/- 2 years is probably still good enough.
It looks like MTI (maker of the "safe-t-alert" suggests replacement every five years.
http://www.mtiindustries.com/PDFs/rv/35_Series.pdf (scroll to the very bottom.)
https://www.amazon.com/MTI-20-441-WT-Surface-Designer-Detector/dp/B000FLLWGA/ $45 for a replacement isn't so bad for a safety device.. I do know a couple people who have come close to blowing themselves up in an RV, too.. so I strongly suggest having one of these things.
Personally, I also bring a battery powered smoke / CO detector along when I use my pop-up camper. I'm pretty worried about the 45 year old furnace in that thing trying to kill me too..
https://www.amazon.com/Kidde-KN-COSM-BA-Battery-Operated-Combination-Monoxide/dp/B00PC5S0PA