fishstix

Being full should help. You will still get swings in RH but it shouldn't be too drastic (as is your case). Is there any way you can check the seal of the unit? Sometimes the corners won't align right and you'll have a bit of space. You can fix that by re-aligning the door with the pivot brackets.

you can check it at night by throwing a light/battery lantern or flashlight inside then closing the door. Turn off all the lights and check all around the rubber seal for light coming through.