Don't know what type of water heater you have but on my Atwood, the LP lights off a burner and that blows heat through a hollow tube(maybe 3" diameter) that circulates around the water tank, and exits out the front to the exterior. I had some mud daubers build a nest inside the tube and this restricted the free flow of heat through the tube. This caused some of that flame to back feed and fumes could possible be leaking in to the interior of your coach.
Naturally it is also probable that your CO detector is original and since they have a service life of around 5 years, I am sure that yours needs replacing. All you have to do is take the CO detector loose and look at the back side and you will see the expiration date.