Check your grounding. Measure from the DUT to a ground rod you drive into the soil. Not knowing your soil composition, I would at least start with a minimum per code requirement in your area. To drive the rod, make a "water" driver with conduit and a hose attachment and "drill" the hole first, then insert the ground rod. While the ground is wet, use your meter to test the DUT case ground to your new ground. Any differential voltage is a problem and this needs to be solved first.
If your grounding is good, then move on to the other issues. Water intrusion from high humidity can cause these issues too. If this is at your demarc, then temperature differential can cause this too. Like the others have said, the flooded cable is best to prevent water intrusion, but you might need a desiccant, heater, or something like that in that connection box.
If these buildings are not used all the time, do they turn off the power and let the temperature and humidity go "out of control"?