1. I am NOT positive that the 510 has the defrost timer in the control board. The Norcold time is automatic but the time is every 48 to 54 hours when left ON continuously, the refer shuts down for 2 hours to help defrost the evap fins, but you state 30 hours is the problem time
2. I would run the refer and every day once a day shut it OFF for 10 seconds and restart and see if you still experience the raise in temp.
3. Another thing I would do at the shop is wire the 120 direct and install our USB temp recorder and let it go for a week. The temp recorder can be set up and we set for 15 minute temp logs. Doug