[MidMOTraveler]

Your smoke detector was just a stocked item st the factory.
Keystone probably had pallets of them sitting there. They just are out of date batteries, replace it. This should be done 2 times a year anyway.
FYI the 2019 model Montana’s started being sold around July so your 2018 has been around awhile.
Enjoy your new rig and welcome, there lots of good information here.