RVs handle this with a device called a converter.  It runs all the 12v stuff when you are
connected to city power, and also charges the batteries.  When you disconnect the city power,
it goes back to running the RV from the batteries.  They are about $200 at places like
www.campingworld.com.

Bill

----- Original Message ----- >
>  While I am at it, I want to wire the circuit so that when the fridge is running off 110AC,
the fans would be powered by the outside power instead of the battery (maybe a 12VDC wall
transformer pluged into the outlet inside the cabinent to the right of the fridge).