Did you check on the back of the unit you took out, it has the mfg date. Bought my motor home April 2011, detector was made 2010 and says on front to replace 2014. I have the same issue, does not go off during day, 1hr after all quiet and in bed, it fires off with blinking red/green, which states low voltage or bad unit. PS you can order unit from Amazon or Ebay, for $100. plus or minus a buck or two.
Logic will carry you from point A to point B. Imagination will carry you everywhere. "Albert Einstein". 2011 Georgetown 360, Bride of 45yrs, and 2014 Chev Equinox toad, what more could you ask for.