I recently ran into a similar situation with my Norcold 2 way, wouldn't get cold enough on propane, but on 110 it would freeze everything when set to the middle temperature position. I called Norcold and one question that came up was the clearance on the sides and top of the fridge in the compartment it was in.
Seems the manufacturer (KZ-RV Inc) neglected to insulate the compartment, or fill in the 4" gaps around the box. So when on electric, the heating element was inside the insulated chimney and worked fine, when switched to gas, the burner which is below the chimney, generated so much heat that the cooling unit inside couldn't keep up, and worse as the unit called for more cooling, the burner would put out more heat, defeating the cooling of the system.
So this weekend I'm pulling the fridge, insulating and closing off the 4" wide gaps around the fridge, and should see a huge improvement in performance.
Also there is a bulb in the back on the fins, usually with a metal clip. if the fridge is n't getting cold enough, you can move it up to trick it into getting colder, and down if it's getting too cold. Move it only about 1mm at a time, more than that can create more headaches as it will swing the temperature too far in either direction.