Remove all 12v fuses from the distribution panel;
Take a multi-meter and put in the mode where you can see if
a circuit is drawing a current (usually no more than 10A);
Test each fuse and find out which circuit is drawing the current.
Now trace back to the appliance; I had the same issue and it turned out
to be the smake/CO2 detector and the fridge (even if it was off);
My batteries would drain in < 2 days;
Also - good point about the break away switch
Gerry