The only problem we have had on temp swings is that one of us forgot to check the flap that seals between the 2 refer section doors. That thing does to close properly, never has and I have seen a more than a few posts on it. With the flap open the doors don't seal and weren't the wild temperature swings your discussing. Close the flap manually and the temp stays a 34 even in south Texas in the summer time with the sun shining on the side of our rig. The refer is fine once you learn to check that the door closes properly.
JimF, CWO, US Army Ret, 100% DAV
2011 Georgetown 378TS
2008 Dodge Caliber Toad