You ALWAYS wire the 120 element DIRECT to 120 power to test a CU. Just 2 wires and connect to an extension cord. Run 24 hours. IF the FREEZER does not get cold or below 10 degrees the CU is bad. IF the lower section does not get below 38 degrees the CU is bad. Now, I am not a fan of rebuilt as a Service Shop. IF they are defective they do not pay a fair labor for the shop to diag and replace the defective unit. Now, IF or you are doing the work yourself, I would go with a Rebuilt. That way, IF it is bad, you have the time to replace the defective unit. BTW, Dometic and rebuilders BOTH have replacement CU's for your model. I would never trust that so called RV shop again. Doug
PS, I don't know where that poster got his info that a defective Thermister will freeze or get extremely cold. WIVES TALE. I have yet to see a Thermister make a BAD CU freeze or get extremely cold. Yes, they can cause too low temps, but don't use that as a guideline for Diag on a cooling problem.
