Refrigerator temp fluctuates too much!!

This summer I've noticed our frige seemed to be not as cold. It is a 12 year old dometic rm2652 americana. It will get down to about 38f overnight but by late afternoon will be at 52f. I know the days have been hot and opening the door alot will cause it to warm up. After one of our trips I kept the frig on(makes no difference gas or AC) and didn't open the door once all day. By evening it had gone up to 50F. Should a frig fluctuate that much even on a hot day? I disconnected the thermistor at the outside back of the unit. Overnight the frig got down to 26F by morning but went back up to 46F by evening without opening the door once. It was a hot day but I don't understand why it can't stay cold. I put a fan by the coils in the back of the unit and that seemed to help a little(2* cooler). I'm not sure where to go from here. No leaks. No ammonia smell. Any of you have any experience like this? Am I asking too much from my little 6cf refrigerator??