I too have a Dometic refrigerator with a door heater that cannot be turned OFF. Initially I was very disappointed with the quick discharge of my battery when boon-docking, even though I was very careful with my power usage. I discovered that my refrigerator was responsible. I have installed a switch to disable the heater, and the battery life has been greatly extended. Furthermore, I have never had any condensation around the refrigerator door gasket. It seems the heater is intended to prevent a problem that doesn't even exist.
In my refrigerator model the wire carrying power to the door heater ran thorough the inside light housing. I cut the wire, installed a switch in series, and mounted it in the housing.
You can view this and many other of my mods by following this
LINK