Why does frost keep building up in my freezer?

What is actually going on

Three parts cover the bulk of it: a defrost heater gone open circuit, a thermistor no longer reading the coil correctly, or a door gasket that has gone slack and is letting marine air into the cabinet. The ice then insulates the evaporator, so the freezer loses ground gradually instead of failing outright, which is why the display usually still looks correct.

What we check, and in what order

The gasket is the cause worth checking first in Pacific Beach. Kitchens here run humid for most of the year, and a seal that would be perfectly adequate in a drier inland kitchen admits enough moist air here to overwhelm a defrost cycle that is otherwise working correctly. Close the door on a strip of paper at several points around the frame and see where it pulls out easily. That is where the air is getting in.

The heater and the thermistor are the mechanical half. We test heater continuity and current draw, and we compare what the thermistor reports against a calibrated probe at the same spot. A sensor that has drifted a few degrees will keep the control from ever calling for a defrost, and the control is then doing exactly what it was told to do.

If that matches what yours is doing, say so when you call — it decides which parts come on the van.