I can't speak to Bill's suggestion about contacts on the Norcold. The only problems we ever had with the Dometics on both coaches is when I screwed up and parked too long off-level, decades ago. There were recalls bigtime on Norcolds the last few years, so I'd make sure that had been addressed if it were mine. Additionally, when our old coach refer couldn't keep up with hot days, I installed a 12v/solar fan in the back which made a big difference in cooling ability; today's rigs often come with fans already because they do make a difference. Convective air alone is inadequate these days to evacuate sufficient heat from the coils - forced airflow is much better. You also said your fridge was on the sunny side of the coach, and that can make a difference.
But before I spent big bucks on a new cooling unit, I'd make sure any fans back there were working as designed, like John alluded to. Especially given the freezer seems to work fine.
Joel