Assuming I understand your description it sounds like you are saying the coil has one copper tube that runs the length of the unit, then it returns to the other and so forth. Is this correct? If it is and you are getting frost on one side of the unit instead of the first half of the coil where it enters the evaporator. The your problem is not a refrigerant charge but an airflow issue. If it is the first half of the evaporator then and only then is it a low charge.
Now assuming you have a airflow problem, I would look into if maybe the fans have been replaced. I would look into their specs for horsepower, rotation, fan blade direction, is it where it belongs inside the opening where the metal shrouds around it. Also look at the back of the coil it is possible that it may be restricted from dirt or some food has been placed too high blocking the vents that circulate the air. Remember that the open air units are really closed, not by a housing but by an air curtain. This is void if food is stacked up too high. Ask the employees to show you how they usually stock food on a normal basis & ask them what do they do if they they have a large load of food.
Another thing I would look at is are the vents clogged with dust from inside. Is the plenum blocked or no longer secure against the wall as designed to be. Someone may have worked on it prior & not installed it back as it should be.