[travelin texans]

My LP/CO detector was hardwired to the battery (smoke alarm had a 9 volt battery) if it was fused I don't know where it was as I never had it blow. I did replace the detector that went bad after about 4 years, a year shy of being 5 years old from the date on the back.
As to running the batteries down that's normal as other than that detector there are other items that have a parasitic drain on the batteries.
Your best option is disconnect the negative cable from the battery where it goes directly to ground, this insures no more drains. Or add a true disconnect device to that cable or if it not going to be used for extended storage remove the batteries & place them in the garage with a battery tender attached, be sure to check the water levels regularly (this should be done whether being stored or in use) topping off with distilled water.