Did you know???

Did you measure the propane detector directly, or did you use something like a Kill-A-Watt on the AC plug?  If you measured the AC side, don't forget that there are several other loads.  Trickle charge on the battery (including losses within the converter), circuitry in the fridge, TV (even if off), antenna amp (if left on).  Probably something else that I forgot.

If you are measuring from the end of the AC plug outside, several things I mentioned above are still a factor - even if off.  The converter has an internal loss converting from AC to DC.  I don't know the exact amount, but modern converters loose around 7%.  This loss is carried over to all the other 12 volt appliances so everything uses about 1.07 times more when on AC than when just on battery power.  Plus the converter needs some power to run itself, which boosts the reading at the AC plug even more.  Even if the fridge is off, its circuitry still uses some power.  This is also true of the TV and microwave unless you unplug them (this is commonly called "vampire power", a big problem in homes).  So that 23 watts you've measured is more than the LP detector.