Keeping space between the insulation and the fridge side walls is a good idea. As you likely know, today's frost-free models' sides heat up during defrost cycles, sometimes significantly in my experience with the freezer in the garage and our kitchen fridge, and the heat must be able to vent away. Normally the unit's manual dilineates the manufacturer's prescribed spacing, which may be more than our RV configurations can provide, but we can only make the best of it.
Joel