Drain tube.
Any time a self-defrosting freezer does not seem to be defrosting correctly, the 1st thing I do is suspect a blockage in the drain tube. Ice builds up in the drainage tube, not allowing the condensation a place to escape.
Emptying & thawing out the freezer normally does the trick to clear the ice blockade. SOMETIMES, simply pouring a bit of HOT water into the drainage tube will also do the job. A piece of wire can be used to help dislodge the blockage, but be VERY VERY careful not to poke a hole in the tube.
With a residential/commercial freezer, a quick check of the drain pan can varify the problem (a dry dusty pan is not draining correctly). Obviously, this will not work with a unit that has no pan...and many RV fridges just drain outside.