FYI, 310 is the Model of the Jack system. While the slide mechanism is HWH that is not the model of the slide system. Your HWH slide system is one of the easiest to diag and fix. The fact that CW and some other shop could not find and give a diag in less than 1 hour shows the state of incompetence in the RV industry service industry. If the jacks and other slide operated and just the one did not retract, that points to the solenoids(2) for that slide not operating. There is a extend solenoid and a retract solenoid. EITHER could cause a retract problem. BAD valve(not releasing electrically) or 12 volt supply to the solenoid problem. Good news is, a GOOD shop can get hold of HWH and get diagnosis help. HWH prioritizes authorized shops for Phone technical help. Doug
Use this link for the service manual.
https://www.hwhcorp.com/ml12771.pdf