Can you be specific as to which fuses blow, meaning to which circuts? I ask because it doesn't make sense that if the short was in the detector wiring that it would cause fuses to both the house and chassis batteries to blow. Still, without knowing the circuits the fuses are protecting, it's hard to give advice on a way to solve the problem. I am not familiar with your coach wiring so it may be possible the detector is connected to both power sources.
Also, when did the detector "die?" Was it before or after you replaced the house batteries?
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.