Freezer has not reach down to zero

You are not using the correct tools. The Heat Element for your model is a 300 watt 2.4/2.6 amp Heating element. Heat Elements either WORK or they do not heat. There is no partial heat from a Heat Element. Freezers in almost ALL RV refers are NOT controlled by the tstat. They will get below 10 degrees regardless of the tstat setting. The operational range of a RV refer freezer is 0 to 10 degrees. Most will get slightly below zero, but that is rare. The reason the freezer does NOT need regulation is the design of the cooling unit. The extreme cold for the refer happens at the top and then goes down to the lower section and then is regulated. Has any work been done on the refer? Doug

PS, 40 degrees is NOT good. It should be below 36 degrees. I suspect a partial blocked Cooling Unit.