Here is a link to the repair manual for your refrigerator:
http://www.dometic.com/QBankFiles3/E...nual_17606.pdf
On page 4 is a list of probable component failures for a "freezing" refrigerator: I'd suspect that since you've already attempted to move/adjust the thermistor on the cooling fins, you may have a bad thermistor and you can get a replacement at most RV dealer parts stores. The next most likely failure would be the lower control panel. It's the "control board" that's located outside, in the lower vent space, under the black plastic cover. My suggestion would be to not purchase a Dometic replacement, but to order the Dinosaur control board. Dinosaur Electronics produces aftermarket control boards that are significantly more reliable than the OEM components. Pricing is competitive so cost shouldn't be a factor in improving your reliability.
As for "what happened" ??? Who knows, but if it's not the thermistor, then it appears that your refrigerator is in the "limp" mode and the control board is calling for continuous cooling because of a circuit failure on the control board. To ignore the problem will likely cause your refrigerator cooling unit to fail because of the continuous operation of the heating unit.
Also, as an unrelated side note: Realize that absorption refrigerators work by heating the sulfur/ammonia mixture which turns to gas and then condenses to absorb heat from the inside of your refrigerator box, causing it to be cooler than outside. That process, heating the sulfur/ammonia must occur "in the chimney" which is exposed to the outside elements (behind the lower vent opening Once the ambient temperature gets to a "certain point" the gas flame and/or the electric heating element can not produce enough heat to cause the mixture to turn to gas and the refrigerator operation will cease. That "problem" will occur once the outside temperatures drops below about 15 or 20 degrees. Each refrigerator is an individual unit and some may work in temps down to 10 degrees while the one in a trailer sitting beside it may stop working at 15 degrees or may continue to work down to 5 degrees. So, don't expect your refrigerator to consistently operate once the cold weather drops below freezing and remains there. Some people have better "cold temperature luck" than others, but there's a "bottom line temperature" below which your refrigerator will cease to cool.