We've had one, too, since January 2003. I moved it to this Montana when we traded. Phil, you're gonna love that thing. Two tips for you, if I may. First, when the light stays on fulltime rather than flashing, that apparently is the signal that the batteries are low. I think it only does that when it would otherwise be flashing. Our batteries last 12 to 15 months.
Second tip - if yours is mounted in the bedroom like ours is, that flashing light is bright. It doesn't wake me up but it does keep me from getting back to sleep. A heavy sock fits nicely over it! I could always go out and flip the regulator switch but I've had this thing start flashing when it's five below zero outside and I'm in my jammies. Going outside is guaranteed to wake me up so the sock is a workable alternative until morning!
By the way, this works with the style of Marshall regulator on our 2007 Montana and the earlier ones. At some point Keystone moved to a different regulator and this won't work with them. The regulator it works with looks like this. When you remove the black cap, there is a small hole in the top of the clear cylinder. The plunger for the new piece goes in that hole. If yours doesn't have the hole, it won't work.
Also, this detector/flasher thing is made by Marshall Brass, the same people who made the regulator, so there's no concern about removing the original black cap, etc.