It sounds to me like the first/easiest place to start it to check the electrical connections of the 120VAC heating element. As others have said, a bit of oxidation/corrosion on the terminals can result in a poor mechanical connection and cause problems. Second, measure the resistance of the heater.
Here's a link to a service manual which may be of help (assuming your fridge model is included):
https://fourwheelcampers.com/NewDometicRefrigeratorManual.pdfPage 10 has a table with the resistance values for the AC heater element for each fridge model.
