I mis-stated my point on this one..The problem happens when dealing with a smart charger that shuts completely off once the batterys are fully charged.. If the charger floats or is an un-smart one, then using it as a power supply is not really a problem.. Sceniaro for some (not all) smart chragers is: The charger fully charges the battery then shuts off.. When the fridge kicks on, The volts drop enough to tell the charger to charge.. so it kicks in and looks at the amperage it is pushing.. with the fridge on, it thinks the battery is really discharged because of the higher ampere draw and starts to push some more amperes into the battery by raising the volts.. fridge shuts off and the charger ramps down slowly, thinking it is dealing with a battery in need when in fact the battery is now getting overcharged. The manual that came with my Xantrex Truecharge cautions to not use it as a power supply for things like refrigeration.. I have seen the Truecharge playing strange games while running the stereo .. not a high draw device.. The Truecharge has a pushbutton that makes it act like a power supply but they caution to disconnect it from the batterys when using it that way..
Base case, if ya have a plain old charger, probably not a problem.. if ya have a smart one that shuts down the charge when full, ya might want to look closely at wiring in a power supply to run something like a fridge.. Whew.. Time to go watch football now..