MOst cases I am familiar with work into the 50-55% range at store temps not exceeding 75 degrees. Remember Relative Humidity is just that....Relative. At a higher temperature the same RH actually means a higher moisture content in the air.
If the open freezers you are talking about are multi-deck freezers and you are having trouble with excessive frost build-up, try looking at the ambient and secondary air bands. Make sure they are in good working condition. This is how the store air is kept out of the freezers in the first place. If these are not working because of fan motors out or dirty honeycomb grills, this could be your problem.
If the open freezers you are talking about are coffin cases, look for air infiltration from sources like supply registers from the a/c system.