Freezer fan seems to be working, gasket is tight.
There's no ice buildup in the back of the freezer compartment. (I'm assuming you meant on the inside of the freezer, like behind the ice bin & a plastic shield over the light bulb. If you meant the back as in behind the whole refrigerator, I can't tell...it's built in & I didn't undo that)
I cleaned the dust as described in the user manual, but there wasn't much there. (This is behind a cabinet-like screen above the refrigerator, where the mechanical stuff is located.)